D10 IS A MORE FUNCTIONAL, INTUITIVE, AND ELEGANT SYSTEM THAN D20.

Note that I am NOT saying that d10 is more 'realistic', but only because that connotes, in my opinion, that d10 can't be as fantastical or legendary as d20, and I'll hear none of that. It is a more accurate model for describing a reality, but I'm hesitant to use the word 'realistic' because d10 has the same potential for running epic adventures as d20.


Where to begin...


ROLLING DICE

d10 only uses 10-sided dice. There are basically three different kinds of rolls: skill rolls, trait rolls, and damage rolls. Each of these is done by rolling a certain number of 10-sided dice and adding up some (perhaps all) of those dice. All rolls are in the format [a]k[b], where [a] is the number of dice rolled and [b] is the number of dice used to determine the result (the number of dice 'kept'). The 'k' represents 'keep'. For example, you can say "roll 5k3," which would mean "roll 5 dice, keep 3 of them," where 'keep' means 'use only the results of'.
Skill rolls are [skill] k [trait]. Trait rolls are [trait] k [trait]. Damage is a little more complicated, but still fairly straight-forward: use the base [roll] k [keep] values of the weapon, and roll an additional number of dice equal to your Strength.
d20 uses six different dice: a 4-sided die, a 6-sided die, an 8-sided die, a 10-sided die, a 12-sided die, and a 20-sided die. For each roll, you have to know which of how many to use. There are no real generic rules for this - anything from damage, number of opponents affected, durations of spells and other effects, etc., use their own specific combination of a number of different-sided dice, and you have to know them. The notation used is [a]d[b], where [a] is the number of dice rolled and [b] is the number of sides that die has, so 4d6 would mean "four 6-sided dice."

Making an attack in d10 sounds something like this:
Accuracy: "Okay, I have 4 ranks in the Axe skill and my Agility trait is 3. An attack is a skill roll, so I'm rolling 4k3. But wait, I'm using a magical frost axe that lets me roll an extra accuracy die, so it's 5k3: 5 dice rolled, adding up the values of 3 of them. I have two attacks, so I do that twice."
Damage: "The base damage on an axe is 1k3. My Strength is 3, so I roll 3 more dice, so it's 4k3. Oh wait, the axe I'm using deals +2k2 frost damage, so my roll is actually [4+2]k[3+2], or 6k5. That's 6 dice rolled, adding up the values of 5 of them."

Making an attack in d20 sounds something like this:
Accuracy: "Okay, I'm an 8th level Rogue, which means that my Base Attack Bonus is 3/4 of my level, which is +6. Oh, wait, for every 1+5n Base Attack Bonus, I get another attack, so I'm at +6/+1. My Strength is 17, so the bonus I get from that is... 17 minus 10 is 7, and cutting that in half is 3.5, rounding down is 3, so my Strength bonus is +3. So that's +9/+4 so far. The axe I'm using is magical, so I get an additional +1 to accuracy, and I have Weapon Focus with the axe, so it's another +1. That's +11/+6. Okay, so now I roll 1d20 and add +11 to that number to see the result of my attack. Oh, I rolled a 19. What's the critical threat range for an axe? Looks like it's just 20. Let's see, do I have the Improved Critical feat for the axe? No, but the axe also has the Keen enchantment, and that doesn't 'stack' with the Improved Critical feat anyway. So the critical threat range is increased from 20 to twice the number of numbers in that range, so the final range is 19-20. Okay, so I've made a 'critical threat', and now I need to 'confirm' the 'critical threat', so I roll the d20 again and add +11 again. Okay, looks like I hit the enemy's AC again, so I've just 'confirmed' a 'critical hit'. Okay, now for the second attack..."
Damage: "Alright, so my first hit was a 'critical hit', and my second hit was just a normal hit. Let's see... the damage dealt by a battle axe is 1d8. To that, I add my Strength modifier, which I've already calculated was +3. But wait, I'm using the axe in two hands, so it's actually 1.5 times that bonus, so it's +4.5, which rounds down to +4 from my Strength. The axe is also magical, so it deals +1 damage, and +1d6 cold damage. I also caught my opponent off-guard, and so I deal my extra Sneak Attack damage because I'm a rogue, so I also roll... let's see, how many Sneak Attack dice does an 8th level rogue roll... +4d6. Okay, so it looks like my roll is 1d8 + 5d6 + 5. Oh, shoot, wait, it was a critical hit, wasn't it? Okay, what's the multiplier on an axe... x3. So let's multiply everything by 3: 3d8 + 15d6 + 15. Wait, no, the axe was a Frost Burst axe, so it follows special rules for critical hits. It doesn't deal +1d6 damage anymore, but instead deals +1d10 if the weapon it enchants has an x2 critical hit multiplier, +2d10 if the weapon it enchants has an x3 critical hit multiplier, and +3d10 if the weapon has an x4 critical hit multiplier. My axe has an x3 critical hit multiplier, so I deal +2d10 damage instead of that +1d6, so now we're down to 3d8 + 2d10 + 12d6 + 15. Wait, no, the damage that the Sneak Attack deals is not a type of damage that multiplies, so the final roll is 3d8 + 2d10 + 4d6 + 15. Okay, so I roll three 8-sided dice, two 10-sided dice, four 6-sided dice, add all of those up together, and then add 15 to that total, and I get the damage dealt with my first attack. Now for the second attack..."

As you can see, d10 rolls are far more straight-forward, and there are far fewer things that don't follow any sort of pattern that you "just have to know." On top of that, in d20, the number of bonuses you can get from different sources is quite large: you can get ability score bonuses, armor bonuses, natural armor bonuses, shield bonuses, cover bonuses, size bonuses, deflection bonuses, luck bonuses, morale bonuses, dodge bonuses, magical enhancement bonuses, racial bonuses, and there's even such thing as 'unnamed' bonuses. Some of these 'stack' with each other, and some don't - you "just have to know" which ones do and don't. Beyond that, there is such an astoundingly large number of things that can affect a single roll minutely that you can never really be sure that you're making a roll correctly. You could be forgetting the "-4 to attack an opponent if you are using a ranged weapon and he is not immediately next to you but is prone" modifier, or the "+1 to attack for using a ranged weapon from the elevated platform that my horse counts as, unless the opponent is attacking my horse in which case my opponent and my horse are technically in melee and so I receive a -4 penalty for firing into melee unless I have the Precise Shot feat" modifier. This can seriously end up slowing down a game unless the players and GM are aspiring accountants. Hell, I bet that some of the internal calculations performed by some computer games are simpler than many d20 rolls.


ABILITY SCORES

I'm going to use "ability score" and "trait" interchangably.

There are, in my opinion, two reasonable ways of handling ability scores. The first is making everything extremely general, which is useful when each player is controlling masses of people, and something a lot more specific, which is an excellent way to offer a player plenty of variation to make his character that much more interesting.

The first involves three traits: Body, Mind, and Soul. You use the Body trait for all things physical, the Mind trait for all things mental, and Soul for things that fall outside that category. In this sort of system, which I'd actually be pretty reluctant to use because of its brutal simplicity, I wouldn't even include "Soul," since things like spellcasting and internal drive can easily be represented with the Mind trait. Then again, that could just be the heathen in me.

The second method is by far more preferrential if the player is controlling a single character and wants to say something more complex than, "his body is in better condition than average." For example, a player needs to be able to reflect in his character's ability scores that his character is, "nimble, dodgy, and mentally alert, but impulsive and physically frail." How do you say that with just Body and Mind? How would that ever manifest in the action rolls the character makes, unless the Game Master was to take explicit care to interpret his rolls differently on that pretense - but that's the job of the game mechanics, so the fact that the GM needs to do so in order to properly take into account these aspects of the character is a sign that the system you're using isn't a good one.

(There's actually another method that many computer games take, and that's to divide every action your character can conceivably perform into its own trait or skill. An example of this is a game that requires any character who wishes to fire a bow to have ranks in all of Bow, Archery, Fletchery, Pulling, Spot, Hunting, Ranged Weapons, and Springy Devices That Snap When You Let Go Of Them, as well has having supremely superhuman strength and dexterity - we're talking ten or eleven standard deviations above the norm.)

For the second method, I very much like the usual breakdown of traits used in Legend of the Five Rings (L5R), but independent of this "elemental ring" business. As an aside, I think the organization into elemental "rings" is an excellent idea - in the L5R campaign setting - but I'm only talking in general terms here so I won't include them.
So, the breakdown I like is:
Strength
Agility
Reflexes
Stamina
Intelligence
Willpower
Awareness
Perception
Void (which I've been known to call everything from "Charisma" to "Soul" as well as "Void")

This, to me, makes sense. I think this does the best job of describing the general breakdown of physical and mental aspects that a character can have. You can divide it into more, but that would be nitpicking, and you could consider combining a few, but... well, we'll see what happens when we start combining them.

Before I continue, I also want to stress the point that ability scores are not all of perfectly equal value. This is in fact an unreasonable thing to say, because any given trait can be far more useful than the rest depending upon culture and immediate circumstances. For example, having a high Strength would help you pave the way to success in an ancient, war-like setting, but Intelligence is much better a road to wealth and happiness than Strength in the modern world. Please do not have any silly pre-conceived notions about all traits having to be equal, or some similar garbage. Each is meant to represent an aspect of a human being that is meaningfully different from the rest, and the focus is NOT on dividing the realm of human ability into an arbitrary number of chunks that are each of the same influence, because that would just be silly.

Anyway, here's why the previously mentioned breakdown makes sense to me. Let's talk about the four physical traits first.

STRENGTH: Strength represents how hard you hit, how much you can lift, and all the usual stuff that you think of when you think of a strong person. I rarely run into trouble when talking about Strength. Strength does not overlap with Agility because Strength wouldn't determine in the slightest whether or not you are actually able to move precisely, and only governs the intensity to which you can influence objects; Agility is used when writing caligraphy, and Strength would only ever get involved if you were stabbing the pen through the writing surface or breaking the pen. Strength does not overlap with Reflexes because being strong does not entail any degree of quickness or dodginess (other than being able to sprint faster, but that's not something really relevant to Reflexes). Strength does not overlap with Stamina because being strong does not entail a high endurance. Plenty of joggers, for example, have a very high endurance but are otherwise no better than the average person at lifting or carrying something, and plenty of weight-lifters can only perform at their peak for no longer a period of time than the average person; a sprinter would be an example of someone with high Strength but average Stamina.
AGILITY: Agility represents coordination, and also deals with fine motor control. You would use agility for everything that predominantly requires good coordination, from acrobatics to lock-picking to landing a round-house to someone's neck. Agility does not overlap with Stamina because... well, they're completely different; Stamina is a measure of how long you can do something while Agility measures your ability to act with precision and coordination. And for the trickier one, Agility does not overlap with Reflexes, because Reflexes doesn't deal with fine motor control and coordination as much as it deals with the ability to quickly respond, physically, to a stimulus. An example of someone with high Agility but low Reflexes is a decrepit old man who happens to be a brilliant watchmaker, and an example of someone with low Agility but high Reflexes is a very skittish, nervous wreck of a man who is constantly bumbling into things, is generally a klutz, and is probably more of a danger to himself when wielding a sword than to his opponent. There is a very minor case in the proposition that Agility be divided into more localized coordination, such as Fine Coordination versus General Coordination (since the aforementioned watchmaker would probably do a lousy job of landing a punch in a brawl, regardless of how good he is at very fine motor movements), but I think this distinction is made effectively enough with skills - if the old man used to be a kung fu master, his training will obviously do much more help for landing that punch. The old man's decrepity (?) is still evident because he would receive a penalty on all physical traits, including Agility, when he reaches a certain age.
REFLEXES: Reflexes measures how quickly you can physically respond to new circumstances. Someone with high Reflexes will be the first to bolt from his seat at the sound of an alarm and would be very good at avoiding objects moving at high speeds, or even intercepting them if he is coordinated enough (see Agility), but Reflexes alone does not entail this kind of coordination. Reflexes does not overlap with Stamina for obvious reasons.
STAMINA: This is self-explanatory for the most part, and is otherwise explained pretty well in the description of Strength and how Strength is different from Stamina.

The only problem I can readily spot is that Agility can be effectively divided into two traits, one that deals with fine motor control and one that deals with coordination in terms of the body in general. An acrobat, for example, will not necessarily be good at lockpicking, and someone with excellent penmanship may not be able to dance very well. I think that this is mostly dealt with through skills, though - the former person has ranks in Acrobat but not in Lockpicking, and the latter person has high ranks in Caligraphy but not in Dance.
Other than that, none of these traits describe two different things, and none of them overlap.

Now let's talk about the mental traits. This is more abstract, obviously, so it may require more thorough explanation.

INTELLIGENCE: Intelligence measures cerebral quickness, the ability to understand things comprehensively, and to think critically. Having a high Intelligence will help you in discussion of abstract concepts and in analysis. Intelligence does not overlap with Willpower because having a high Intelligence does not make one any more able to tolerate hardships, to withstand emotional abuse, or handle stress. Intelligence does not overlap with Awareness because it does not make someone any more aware of their surroundings or grant the natural ability to naturally understand others' perspective without a focused analysis (this will be explained more in the description of Awareness). Intelligence does not overlap with Perception because having a high Intelligence does not entail having any sharper or keener senses.
WILLPOWER: Willpower is essentially mental endurance. You use Willpower to resist fear and compulsion, to withstand pain, and to weather emotional abuse. Willpower does not overlap with Awareness because Willpower deals only with mental resilience, and not at all with social grace or having any thorough understanding of one's own circumstances in his surroundings. Willpower does not overlap with Perception for reasons that should be obvious.
AWARENESS: Awareness is rather difficult to explain. Awareness is the understanding of one's own circumstances in his position, and the quick, accurate appraisal of one's surroundings. This understanding is not on any sort of academic or analytical level, but on the level of social and emotional dealings as well as quickly understanding what's going on in a chaotic environment. This does not overlap with Perception because Awareness is more of a comprehension, whereas Perception focuses purely on precise physical (or otherwise sensory, such as aural or olfactory) attributes. Awareness does not overlap with Intelligence because a high Awareness does not say a thing about academic potential, analytical thinking, critical thinking, or understanding of complex subjects, and Intelligence has nothing to do with empathy, social grace, quickly ascertaining who is doing what in the middle of disorganized combat, or cold-reading people they interact with. Examples of people with high Awareness but average Intelligence are self-proclaimed psychics and related magicians, professional athletes, and people with a tremendous capacity for empathy who are otherwise lost on complicated subjects. Examples of people with high Intelligence but low Awareness are the socially awkward geek stereotype, an absent-minded professor, and a brilliant young enterpreneur whose intelligence is superceded only by his abrasiveness. An argument for combining Intelligence and Awareness would owe an explanation of why professional poker players wouldn't last a week in medical school.
PERCEPTION: Largely regarded by powergamers as the most useless trait, Perception describes the sensitivity of your senses. It's closer to a physical trait for this reason, but it's still considered mental for the most part because this trait doesn't describe any movement of any physical portion of a person's body. Even so, Perception can be very useful for identifying people at a distance, reading from a variety of awkward positions (such as hanging upside-down behind someone reading a letter), noticing secret doors, locating traps, detecting forgeries, and its most classic application in gaming: spotting an ambush.

As with the physical traits, I can only identify one problem: memory. Where does memory fit in? You'd think that it fits in with Intelligence since nothing else can really describe it, but I know plenty of people who are very intelligent but have woefully disfunctional memories. I think this is best solved by basing Memory off of Intelligence and offering a "Poor Memory" disadvantage that the player can choose at character creation (as there is already a "Perfect Memory" advantage).

VOID: It's always useful to have some mystical ability score that doesn't entirely fit in with the rest, especially if the campaign setting includes anything that is effectively magic. Void can be used to represent things like hope, and can be relevant to soul-sucking goodness. This is really left up to the GM to decide, based on the campaign setting. L5R has a concept called "Void Points," which allows you to, a number of times per day that depends upon your Void score, summon the drive to perform an action outstandingly well, with your best effort. I can understand why this would be the job of Void if the elemental "rings" organization is used, but in a game without supernatural elements, I would not use Void and turn the "Void Points" into "Willpower Points."

Okay, so that's the breakdown for what I think is the most reasonable division of physical and mental abilities of a human being.
d20 has decided that it doesn't like 8 ability scores. d20 has made the following changes:
Strength: no changes
Dexterity: combination of Agility and Reflexes
Constitution: the same as Stamina
Intelligence: no changes
Wisdom: combination of Willpower, Perception, and some aspects of Awareness
Charisma: the parts of Awareness that aren't covered by Wisdom

...what the hell happened here? We had a perfectly fine, functional system, and now it's all gone to Hell.
DEXTERITY: Not only was there an issue about Agility being split into two separate traits, we're now combining it with Reflexes as well? Every one of the problems we had in describing those with mixed Reflexes and Agility comes up; we have acrobats who have excellent penmanship, we have watchmakers as nimble as ninjas, and we have bullet-dodging painters. Dexterity causes too many problems, and is far less functional than just having two separate traits.
WISDOM: In d20, Wisdom is used for Will saving throws, for skills that help you determine, for example, whether or not someone is lying, and also represents how sharp your senses are. Here we have not two, but THREE traits shoved into one. Lumping Awareness and Willpower is just plain wierd, as they have little to do with each other... but d20 also tosses in Perception? How the hell does that work? How do you have a single mental attribute that helps you do ALL of resist mental stress, quickly ascertain what everyone around you is thinking, and... see better?
CHARISMA: Every social aspect of Awareness was stripped from its foundation and thrown into Charisma, which also, by the way, measures a character's attractiveness. You heard right - ugliness equals social incompetence. d20 has proclaimed that it is impossible to be both attractive and shy, or both ugly and social; it's a contradiction of terms, and is logically inconsistent. (In d10, this is much more sensibly accomplished with (dis)advantages such as Ugly and Dangerous Beauty.)

This is not functional. d20 mashes together, seemingly at random, physical and mental aspects that have little to nothing to do with each other, effectively claiming that if you are good at one that you are good at the other - that if you have good vision then you are emotionally resilient, for example.


HOW ABILITY SCORES ARE USED

Quick quiz: How should you determine how hard it is to hit you? Should it be based on how good you are at avoiding objects coming at you (Reflexes, in d10), or should it be based on a combination of how much fine motor control you have PLUS how well you can do the robot PLUS your ability to throw things accurately (Dexterity, d20)? The creators of D&D sure thought that it was the sum of these latter things, each of which are completely unrelated to actually dodging things that come your way, that would actually be relevant to dodging things coming your way. I guess using the obvious answer ("good reflexes help you avoid getting hit") was too boring for them. They also did a few other wacky things, like having your ability to tell when someone is lying to you be based on the same thing responsible for good eyesight and wax-free ears. It's also the same thing you use to make your Vulcan Nerve Pinch more powerful. What's next, a winning smile makes you better at combatting undead? Oh, wait, that's right, Turn Undead is based on your Charisma, isn't it...

Let's talk about how armor works. Armor is hardened material that you pretty much can't hit through no matter how hard you try. There are some supernatural and/or epic exceptions, but that's how a straight-forward combat system should treat armor. The problem with armor is that it can't both protect every weak spot on your person and allow you to move. For this reason, armor has flexible spots that are armored less that allow you freedom of motion. If you get tired easily, you can use lighter armor, but that obviously can't protect your joints as well as heavier armor.
So how does hitting someone who is armored work? Well, you can thwack uselessly on his armor directly, but that really doesn't do you much good (unless you're using a bludgeoning weapon, which, by the way, is another thing that d10 takes into consideration and d20 doesn't: bludgeoning weapons ignore part of the bonus given to your opponent by his armor in d10, but d20 believes that it's just as difficult to bash someone on the chest as it is to hit him from a specific angle in the elbow). Instead, you focus on hitting more lightly-armored portions of your oppoonent's body. He does need these lightly-armored portions if he is to, like, move, so you can be assured that they do exist. Toward the end of the Dark Ages, what did humans find was the most effective attack to use against heavily armored opponents? Lightly-armored skirmishers with their thin sabers poked right around that armor. This was far more productive than having two heavily armored guys klonk each other across the breastplate with claymores until one passed out from exhaustion and the other took the time to stab him through the faceplate. This takes us to an important question: if you're trying to hit someone in a specific location while they're trying to dodge your hit, which is more important - coordination, or a hard swing? Well, a hard swing is useless if you completely lack coordination. It could be pretty terrifying to see a big muscly guy accidentally catapult his greatsword across the room because he swung too hard and got nothing but air, but actually being coordinated is what's going to land you that hit.
In fact, in d20, you can be the clumsiest, least coordinated buffoon to ever have lived (Dexterity 3 or lower), have a sincerely difficult time putting on your pants in the morning, stab yourself in the forehead with two out of every three attempts to put food in your mouth, but guide your attacks against the nimblest of ninjas with surgical precision, because, for whatever dumb reason, your coordination with a melee weapon isn't actually based in the slightest on your coordination. Imagine that. This even applies to size - you'd think that being able to make your arm go where you want it to go would have any say whatsoever in hitting a small object, but d20 says you're dead wrong.

I'll talk more about combat later. I'm just going to bring up one more issue before moving on to other things: what is with Spot and Listen being skills? Having sharp eyes and elfin ears are purely based on talent - you can't TRAIN yourself to be better at being able to spot irregularities. To "train" these things, you'd have to study every surface or background or environment that you could ever possibly be in, and then place in random places in that environment every single thing that is capable of being hidden, and proceed to play Where's Waldo. That's what you'd have to do to treat your ability to Spot irregularities as a skill. Can you imagine an adventuring party setting up camp for the night, and having the rogue turn to the cleric and say, "hey, would you mind making barely audible sounds so I can practice listening?"
The difference between a talent and a skill is in essence the difference between an ability score and a skill. We should really try to represent something like a talent of spotting things or hearing things as a talent, because it just doesn't make sense as a skill. Hey, isn't that why we have Perception? And d20 has... Wisdom? What does Wisdom have anything to do with good eyesight or keen ears? Even so, having keen eyesight in the d20 system is restricted to a single use, and that's Spotting. Because Spot and Listen are skills, this means that your sharp senses can't be used, in the game mechanics, to help out with skills that rely on sharp senses. If you make it a trait like in d10, you can also base skills that are based on perceiving things on it. Otherwise, you run into the problem in d20 that Read Lips should technically be based in part on Spot.

"Good thing I heard that orc sneaking up on us. And I owe it all to my practice. High five me, holy man."

Can I also ask why the number of skills you have is based on Intelligence? Intelligence can explain how good you are at skills that involve intelligence, but the number of skills really has very little to do with intelligence. Did the creators of d20 think that it's a mental strain to juggle being able to Jump AND Climb AND Swim? Uh-oh, no room in the braincase for learning to Ride a horsie, cuz your brain's stressed enough just knowing how to Jump and Climb, dumby fighter-head. I've known plenty of people who weren't cerebrally quick but had a tremendous amount of skills, and I've known plenty of brilliant, clever, and witty people who were only good at a handful of things, and limitedly so, except for one or two fields of expertise.
The point is that, in reality, the number of things you know how to do is not determined by how smart you are, but how much effort you put into learning them. Only when considering how well you can apply those skills would involve an ability score. The upcoming sections on XP and classes relate to this, as well.


CLASSES

d20 puts way too much focus on classes. In reality, a human being is best described as a unique person with his own set of skills and talents, who also happens to have a class from which he's learned a neat trick or two. As a real-world example, an investment banker is a unique person with physical and mental attributes, skills, and miscellaneous talents completely regardless of whether he actually knows anything about investment banking, and he just happens to also be an investment banker, from which he may have learned a few tricks that he wouldn't have learned if he chose a different profession. He could do any number of things in his spare time, such as performing contract assassinations, or sailboating, and he'd still be able to do those things if he wasn't an investment banker. And even so, what you learn from your class is very specific to the class, and does not govern in the slightest what talents you are then obligated by the logical forces in the universe to avoid attaining.
CHARACTERS IN THE D20 SYSTEM ARE WALKING MANIFESTATIONS OF ROLE STEREOTYPES who happen to have some vaguely humanoid qualities. It should not be like this. The job does not define the person. It doesn't even come close. It is the character who has skill and talent, not his class that makes him so. The character can become good at a class because he has that talent, but having the class doesn't bestow talent upon him.
It's this foundational "bonus" nonsense in d20 that makes classes integrated too heavily with a character's personality and ability. What I mean by "foundational 'bonus' nonsense" is the set of bonuses you get to attacks, saving throws, and skill points, and additional hit points for each level you have in a class. Anyone can give himself a job title and gain any number of tricks from practicing that profession that are specific to the profession, but his ability to actually use those tricks will depend on his own talents, his own coordination, his own understanding of the trick, and otherwise on the time he has taken to improve his own capabilities that apply to the realm of performing all actions. His ability to do that trick depends on him, and him alone, and his professional training merely tells him what he should do and give him some practice at it. For example, a martial artist's accuracy, damage, ability to avoid danger, ability to resist physical assault, etc., is based on his own body. Any little 14-year old punk can learn the same techniques as a full-grown adult, and they should therefore have the same levels in the same class, but the adult will probably be able to punch faster, harder, and more accurately, even though they're using the same technique. A specific class could give you a bonus on certain actions, but d20 gives ALL classes a bonus to ALL of attacks, saves, and hit points that they get completely regardless of whether or not their actions have any relevance to their class. It's no longer the person who is good at doing anything, but the class he has.
d10 defines people as people with their own talents and ranges of skills, and then gives them classes that teach them a useful trick or two. The way the character implements the trick is based on his own natural abilities, as it should be.
(My Latin teacher back in high school has been teaching for more than 40 years, but I highly doubt that he could beat me in a fist fight, ignore poison, or dodge a fireball, and it'd probaby just take one punch in the stomach to down him, not four hundred.)

The Prestige Class is d20's attempt to make up for the fact that Character Classes completely dominate what your character is and is not allowed to KNOW how to do. If you wanted to be something special, something that isn't one of the 11 stereotypes d20 restricts you to, you need to actually NOT be that something for at least six or seven levels, and then you need to seek someone out to train you. So if you wanted to be, say, a swashbuckler, you'd actually have to train in the king's army for several years, and only after that point be able to know any moves that look remotely like fencing. Apparently, every d20 swashbuckler has gone through this process. Want a character who focuses on sneaking around and killing from the shadows, and none of this lockpicking or trap disarming nonsense? Too goddamned bad, if you're playing d20. Pick a stereotype, and go with it for seven levels. Then you can take levels in Assassin, which is the closest you can get, and requires you to be evil anyway (which also makes it unplayable for PC's in most D&D games), and still isn't quite the concept because you probably weren't interested in spellcasting. You could take Ranger, but you'd better be okay with not having any kind of Sneak Attack, choosing a Favored Enemy, and being forced to learn to either dual-wield or use a bow effectively in lieu of what you actually want from the class. Oh, and spells, too. *shrug* Sorry.
Making a swashbuckler in d10 is far, far easier. You can pick any class that has to do with dueling, but you don't even have to do that. You don't even have to pick any class at all, really, and still be fine, though you'd be missing out on some useful fencing techniques (not skill - techniques). Just pump up your Reflexes so you're good at dodging, maybe put some ranks into the Athletics skill so you can tumble and swing on chandeliers, and otherwise focus on melee attacking skills. That's really all there is to it - you're a swashbuckler, and every roll you make will reflect that. You won't be forced to limit your attacking potential or forced to know far more skills than you're interested in knowing, as a d20 Rogue would. Want to be a ninja assassin? You don't even need a class for that, either, though there are usually some nifty ones that give you techniques that maximize the damage you deal if your opponent is unaware of you. You are free to increase how good you are at melee attacks as much as you want, and can purchase ranks in Stealth at character creation as well. If you do want some minor lockpicking talent, you're also free to take that. No worries. You're not, again, forced to limit how much focus you want to put into melee combat, forced to know more skills than you are interested in, or forced to learn how to cast spells. You can do these things if you want to, but you don't have to, but in d20 you have no choice - you MUST follow a stereotype.
In this way, d10 is built solidly on working with creativity. If you can think of it, d10 can incorporate it, and probably without changing any game mechanics whatsoever. On the other hand, d20 punishes creativity, and makes it especially difficult if you want to do something other than make very minor changes to a cookie-cutter character concept. It's the equivalent of saying you're free to make any kind of painting you want, provided you draw one of 11 different objects, and nothing else, and don't stylize it up, and then being told when you ask how you're allegedly allowed to be creative that you can pick the way you sign your name in the corner.
Hell, you don't even need a class in d10, and you can be just as functional and powerful as any other character. You'd be screwed if you tried anything similar in d20.


EXPERIENCE AND LEVELING UP

"Don't worry, guys. Ever since I killed that dragon, I've gotten much better at forging documents." -a rogue in the d20 system

A good way to measure what your character is good at doing after an event that offered a learning experience is allowing that experience to directly increase that specific skill. This should be obvious. A bad way to handle personal improvement is to make the assumption that you've been practicing every single facet of everything you are good at, constantly, and then deciding at some pivotal point that you make a significant improvement in ALL of them, simultaneously and coincidentally. Can you imagine the hell that a rogue would go through every night before he lies down to sleep? While the wizard is up memorizing spells, the rogue has to do every one of 12 different things, from locking and re-picking a lock ad nauseum to doing nothing but backflips for half an hour to impulsively climbing any rough surface he can find to playing hide and seek with the paladin. At least the stuff the rogue has to do is less humiliating than what the fighter has to do - devote a meaningful portion of his day to practicing jumping in place like an idiot and making swimming motions in the air.
A good experience model would give characters improvements in what they actually did, such as by giving XP so that it is "spendable" to increase an ability of your character that relates to what gave him the XP. This is what d10 does - if you get 3 XP from participating in a battle, you may then use that XP to increase one of your physical traits, your skill with your weapon, etc. You may not spend this XP on Knitting, because you did not knit. If you were a d20 character, however, then your Knitting abilities would improve even if you spent the last month escaping from an underground complex completely devoid of yarn or knitting needles. But the d10 experience system allows you to actually say something like, "my character will now spend the next hour knitting," and you are then free to spend the XP you get for completing the session on improving your knitting skill. How is this not more intuitive, straight-forward, and sensible? More importantly, how is the d20 way of measuring improvement not insane?

That's not entirely fair, because the GM can always simply say that a given character has not practiced a skill enough to increase it when he levels up. If, for example, you didn't have any opportunity to learn to attack better, grow tougher, build your mental resilience, or get better at dodging things, then related aspects of your character won't improve. In the d20 system, however, some things increase completely regardless of how little you're offered an opportunity to improve them, such as attacking better, growing tougher, building your mental resilience, and getting better at dodging things. Golly, that list looks familiar! In the d20 system, a wizard who has never swung a sword in his life and who remains completely ignorant of any technique involved in wielding a sword can do better with one than a less experienced fighter. A fighter who has taken to the role of "tank," who has done nothing for the last year other than stand at the front lines like a wall and hit things in his way, gets better at busting out a ninja dodge out of the path of a lightning bolt, even though he's never actually had to ever dodge anything for that time. A rogue who spends his time performing contract assassinations will find in a few months that he's extremely resilient to poison, even if he was never in contact with the stuff throughout his career.

And why is d20 experience given at the thousandths place? When has any amount of XP that was less than 100 ever been of any consequence? You could really just chop off the two trailing zeroes and be better off. This isn't Final Fantasy.

Sure, you can make exceptions for these, but you'd be introducing too many "fixes" to the rules. The point I'm trying to make is that the game mechanics as they are officially described make shockingly little sense. If you need to introduce so many fixes to your game mechanics, you may be better off looking for a system that doesn't need so much fixing.


ARMOR AND WEAPON PROFICIENCIES

Compare the following statements, all of which follow the logic d20 uses to determine weapon and armor proficiencies:
"You are a fighter, therefore you fight well with most every weapon." -Fighter class information
"You are a bard, therefore you can play most every musical instrument well." -something Bards should have if Fighters have the above nonsense
"You are a computer programmer, therefore you are fluent in most every programming language." -yeah, right
"You are black, therefore you can play the bass." -Eric Cartman, South Park

Here's a thought - how about you know how to use a weapon if you made the effort to learn how to use it? In d10, this is obvious, and represented with weapon skills. In d20, this is a novel and sensible (and therefore heathen and unacceptable) concept. It's bad enough that Skip Williams, in writing the Players Handbook, decided to arbitrate which skills a character was allowed to be good at, but I just can't imagine what made him think that there are ANY fighters in history who were equally good with every single one of the weapons he has listed under martial and simple weapons, with only a very minor exception (the weapon for which he took Weapon Focus / Specialization). Wait, that one's too easy, because lots of fighters do actually train with a large number of weapons, even though I still say it's unreasonable to say that they train with ALL of those weapons - I just can't imagine what made him think that there are any BARBARIANS in history who were equally good with every single one of the weapons he has listed under martial and simple weapons. Or Rangers. Or Paladins. I also think it's kind of freaky that all of them are familiar with exactly the same subset of weapons, like they somehow receive identical training with that exact subset of weapons even though their classes are supposed to be different.

Ranger: "You appear to be unarmed."
Paladin: "I am, indeed."
Ranger: "Take this trident."
Paladin: "I have never seen a weapon like this before, as it is not part of my culture and I have never encountered it before now in my travels. I have only ever trained with a longsword, so pretty much all of my life experience is completely useless when it comes to knowing how to use this weapon competently."
Ranger: "Are you a Paladin? I know that I can ask this question and be understood because every single culture that can possibly exist in any dimension in this universe is restricted to choosing a lifestyle among 11 different stereotypes, and that every Paladin has exactly the same training and abilities, again completely regardless of how many lightyears his planet is away from ours, as any other."
Paladin: "Yes, I am a Paladin."
Ranger: "Then you know how to use the trident just as well as your longsword, if not better."
Paladin: "How do you know this?"
Ranger: "I've never seen one either until I picked it up five minutes ago, but I appear to be better with it than the man I took it from, who trained specifically how to use the trident for half of his mortal existence, and I also appear to know every single technique he knows with the trident even though I have never seen any of these techniques even demonstrated."
Paladin: "How convenient."
Ranger: "Indeed."

(As an aside, I'm also highly amused that Skip Williams decided to make Wizards knife-fighting champions. Hell, /I/ barely know how to use a sling or throw a javelin, but apparently all of this comes intuitively to a wizard who has even less experience with those weapons than I do. Hell, most wizards can even THROW KNIVES better than I can (because Skip is convinced that hitting someone with a thrown knife is about as easy as stabbing someone with a held knife), and I actually spent a bit of time learning specifically to throw knives! Honestly now, how is someone capable of throwing knives but not using a shortsword? How can you know full well how to use a mace, but not a warhammer?)

What if you want to focus on a particular weapon? d10 says "put more ranks into the relevant weapon skill." d20 says "too goddamned bad." Other than taking Weapon Focus, there is nothing you can do to make yourself better with a specific weapon (without levels in Fighter, in which case you can make one or two more barely meaningful improvements). d20 has declared that it is impossible to be better with one weapon than any other member of the subset of weapons divinely associated with you upon your birth, and that all extra training with one weapon over another is doomed to be a fruitless waste of time.

As for armor proficiencies... how do you NOT know how to wear armor? That's like not knowing how to wear clothes. Seriously. Armor isn't built with specific techniques in mind on how to wear the stuff - you in fact move just as though you were moving normally, and the armor may limit your range of motion a bit. There are no special techniques to know, no special training to be had to properly operate a suit of armor. You're just putting on thicker clothing than usual - this isn't Mech Warrior.
I know how to wear a thick jacket in the winter that restricts my movements - does this mean that I took the Light Armor Proficiency feat?
(Obviously, putting on armor is a different story. It takes plenty of training to know how to put on a set of full plate. Wearing it once it's on, on the other hand, can't be helped by any amount of training.)

But wait, it's a little sillier than that, even. In d20, if you're "proficient" with armor, then you don't take the penalty from its heaviness and clunkiness on all actions you take. This is intended to represent the idea that you are used to the armor enough that its range-of-motion limitations and its general clunkiness do not bother you. The only problem with this idea comes about when you actually think of what that entails, which should help you realize that this notion is entirely absurd: whether or not you have the patience to deal with a limited range of motion, you still have a limited range of motion. Any materials science class can tell you that raw human willpower has precisely zero impact on the malleability of metal and leather. If you're comfortable in armor, good for you, but you should still take penalties when trying to do something that requires coordination because your coordination is still physically limited.
If I balance a book on my head so much that I become "proficient" with balancing a book on my head in the same way that a fighter is "proficient" with armor, I will still take penalties when trying to tumble if I want to keep that book on my head, regardless of how comfortable I am with the book being there or with sleeping with a book on my head. The penalties are not psychological; it's not like it's claustraphobia that makes you take the penalty for not being "proficient" with armor - it's a physical limitation that can't be overcome with simple wishful thinking.


SKILLS

What the hell is with "cross-class" skills? Screw that - my character will be good at whatever the hell he wants to be good at. God forbid my character be good at something that doesn't contribute to his efficiency as the stereotype of an adventuring role. If I want to play a fighter who is better at knitting than Jumping, and is damned good at both, I can't because the d20 universe has decided that I am a Fighter stereotype and therefore cannot possibly, logically, be more than half as good at knitting as anything a fighter stereotype does. It'd be logically inconsistent to allow it, and the character would vanish in a puff of illogic. I'd be perfectly happy if certain classes required certain skills to take levels in 'em, but not the other way around.
And why do different classes give a different amount of skill points? Why does taking a level of Investment Banker suddenly force me to start learning about completely random skills? If I wanted to become an investment banker in the first place, it can reasonably be assumed (or required; I have no problems with skill requirements for taking levels in a class) that I have the skills an investment banker would use... and now I am required to by the highest powers of the universe to take Knowledge: Tropical Fish because I have more skill points than I know what to do with? I can't take the time that I could have used to learn a new skill to exercise instead? And why does my character's knowledge of Spellcraft suddenly get crippled if I want to take a handful of levels of Barbarian? It's not like you spend so much time learning to do Barbarianly things such that your ability to study Spellcraft is cut in half. Isn't the Barbarian class pretty much defined as someone who is too dumb or lazy to exert mental effort and therefore runs around hitting things? I imagine Barbarians have plenty of free time... so why can't I keep up with Spellcraft as well? Bards can learn Spellcraft while they simultaneously improve their competence with a large variety of musical instruments, not to mention tricks involving Sleight of Hand and lockpicking, so it couldn't be that time-consuming.
Limiting what a character is allowed to devote his time to just because he calls himself a certain title on his resume is a terrible way to let players customize their characters.
The way d20 handles skills is an excellent example of how a character's class completely dominates his physical and mental faculties.


FEATS

The d10 equivalent of feats are Advantages (and Disadvantages, actually; apparently, low ability scores are the only disadvantages a d20 character can possibly have... no bad vision, no defective limbs, no checkered past, no distant enemies, no compulsions, etc.). The purpose of these additional tidbits is to help describe miscellaneous talents that your character has. They are all either features (e.g. Dangerous Beauty, Missing Limb), history (e.g. Good Reputation, Nemesis), resources (e.g. Ally, Blackmail), or other miscellaneous inherent things that aren't something you learn (e.g. Luck). The techniques you learn come from classes, and really not much of anywhere else, though a GM might bend the rules a bit and let you learn one by spending enough XP. Something that could be more accurately called a technique really doesn't have any place in the Advantages list, because it wouldn't make a lick of sense to list them as Advantages, and every bit of sense to have sensei teach them as techniques of their school.
First off, it doesn't make sense for a gaming system to force you into some regular choosing of something advantageous outside of your class and skills (I'm refering to getting a feat every three levels in d20). You should be able to purchase those advantages when you want to, or hold off and save the points you'd spend to improve something different about your character. Secondly, d20 makes a very bad habit of offering what should be class features as Feats. Apparently, Whirlwind Attack, which sounds an awful lot like the name of a technique to me, is something you just wake up being capable of one day. Dodge also sounds like the name of a technique, like something that should be a class feature for rogue-ish characters, but apparently it's an inherent part of a character's personality just like the feats Negotiator and Deceptive. Spirited Charge sounds like a technique that someone who fights from a horse would be interested in, but I guess a child is born with the natural ability to perform a Spirited Charge every now and again.
What sense does it make, honestly now, to clump miscellaneous natural talents like Deft Hands and Athletic with techniques that are only of interest to very specific characters, and then force the characters to pick something out of that mix at a regular interval? What does that even represent, in reality? Getting these advantages at a set interval regardless of who you are makes no sense - like skills, you should get a technique if you work hard enough at it, not by reaching a common, arbitrary level of power. And put the techniques where they belong: in the classes themselves.


HIT POINTS

Have you seen that fake motivational poster with the picture of a warrior who is heavily wrapped up in bandages and has seven or eight weapons sticking out of his chest, with the text reading, "Anything more than 1 HP means you're good to go?" That only exists, and is only accurate criticism, because of d20. In d20 game mechanics, not only are there zero penalties for being injured, but at higher levels it really takes a lot of abuse before a character is meaningfully affected. A friend of mine jokes that the base AC is 10 because every attack automatically makes a called shot to an area that wouldn't affect the target's combat ability if hit. In d20, you are either perfectly fine or unconscious, and there is nothing in between, ever. At least not as a result of being hit.
And again, the bodily health of any given d20 character is dictated by, of all things, his profession. When does that begin making sense? The benefit the d20 character gets to his hit points from being well-built is usually negligible compared to the benefit he gets from his class. It just seems to me that there is another huge assumption made here about the character being "assumed" to be training or something, when in fact not all members of a class, such as soldiers, are equally physically fit. If the party wizard is very concerned about his health and the fighter is a fat, lazy slob, the wizard should take more hits to go down because they're both humanoids, and your ability to tolerate pain is not any better just because you know how to use a wider variety of weapons. If you're told that someone just got punched in the sternum and you're interested in whether or not that would take him down, the question on your mind will be whether he works out or not, instead of whether he's an accountant or a musician or an actor or a computer programmer. Some professions can connote certain physical properties, but those are few and far between - most any profession can include frail and weak practitioners as well as practitioners who are body builders by hobby.
Let's say you're in a martial arts dojo, and the sensei tells you that he can teach you the move Hadouken. You decline the offer and walk outside, where you're shot in the throat with a crossbow. You die, obviously. Assuming that you are shot in the same place with the same crossbow bolt upon leaving the dojo no matter when you leave it, would you have a better chance of surviving if you asked the sensei to teach you the new move? d10 says "of course not," but d20 says you would. In fact, if more random tricks you know, the crossbow bolts you can take to the throat and be completely unaffected, in d20. In d10, you'd have a new way beat the hell out of the guy who shot you, but it shouldn't help you survive the attack.

d10 actually makes players care about being hit (as they should) because taking a hit reduces their capabilities considerably, and each hit usually hurts quite a bit. This fact contributes to the realistic incentive to avoid getting hit. The amount of damage you take is not based on a title you respond with when asked what you do for a living, but on how well you take care of yourself as a human.
Sensible: "Don't worry; he's tough as nails."
Not sensible: "Don't worry; he's a janitor."


SAVING THROWS

There isn't really such thing as a Saving Throw in d10, because you just use an ability check instead. d10 decided that your dodginess should be based on how dodgy you are, that your toughness should be based on how tough you are, and that your emotional stability should be based on how emotionally stable you are. I should hope that I don't need to explain why that makes sense.
d20 went a different route. It decided to base all three of these things not on what would clearly make sense, but instead on your class level. This means that the largest contribution to how nimble you are isn't from your already defined nimbleness (no, that gives only a small contribution), but instead from how long you've been employed or how good you are at what you do best, regardless of your class' relevance to nimbleness. If you're a bard, and a high-level one, then you're more resistant to poison than one who can't play the harp as well as you can. His Constitution could be superhuman, and yours could be cripplingly low, but that wouldn't matter more than how many taverns you've played in.


INITIATIVE AND COMBAT

The d20 initiative system, besides being based on the wrong trait, sucks a huge strategic component out of combat. Previous battle experience and the ability to quickly understand your surroundings apparently don't have anything to do with the speed at which you make a decision about how to act in d20 combat.
Let's say a locksmith is traveling with a hired bodyguard. The locksmith is young and innocent, has never been in a fight in his life, and is generally of the flighty, flowery variety, although he has tremendous talent in making complex mechanical objects because of his steady hands and fine motor control. His bodyguard is a highly decorated war veteran who has spent the last seven years putting his natural tactical aptitude and years of persistent training to use on the front lines in the king's royal army. If these two are walking down a forest path and are ambushed by an orc raiding party, who will have the presence of mind to make a decision about what to do, and who will act first? Clearly, the bodyguard acts first... if you're playing d10. If you're playing d20, then the simpering, scared-shitless locksmith will have the tactical advantage over the bodyguard - he will naturally draw his quarterstaff and assume a defensive position long before the experienced war veteran gathers his scattered wits about him and realizes what's going on, and he is in a better position to react to any actions the orcs take... all because the locksmith has better fine motor control. In d20, all you need to be an excellent tactician is fine penmanship. No, really, that's ALL you need. There is literally ZERO other measure of how good you are at making fast, informed decisions in combat and sizing up your opponents. You'd think it'd at least be a skill or something, but you'd be wrong.
It would, actually, take a less battle-hardened person a longer time to respond to a hostile situation completely regardless of how good their reflexes are. Their good reflexes help them avoid blows, not to gather their wits quickly. If you are either not trained to respond quickly (e.g. the Combat Reflexes advantage) or are generally not very aware of your environment (e.g. you have a low Awareness score), then you will be worse at responding quickly to a hostile situation.

The way d20 implements initiative also removes a sizeable amount of strategy from combat. If your character wins initiative, in the d20 system, he merely acts first. He doesn't have any information about what anyone else is about to do. Winning initiative in d20 is for this reason better modeled as how good your knee-jerk reactions are, and this basically means that there is, by definition, zero strategy involved in combat (if the combat model only pays attention to who is the knee-jerkiest and picks the order of actions based on that, then by definition all actions are unplanned and uncoordinated, and in short lack any sort of strategy whatsoever). But what's worse is that characters who LOSE initiative know what everyone has done by the time they act, so they have a greater awareness of the combat in general! They already know who attacked whom, and therefore have much more knowledge to work with in deciding what they should best do. How badly do you need to fail in modeling combat in order to give the most benefit to the non-combatants in an encounter?
There is a very, very important difference in the way initiative is handled in d10 that makes combat more intelligent and strategic. There are two phases after initiative is rolled: first, each player, starting from the person who LOST initiative up to the person who won initiative must declare which action they are about to take; and then, each player's action, starting from the person who WON initiative down to the person who lost initiative, is resolved. This means that you know more about what is going on if you win initiative and are better able to make an informed decision, and the decision you make about your action is based on what you know of others' actions. Someone who is experienced in battle will trend toward higher initiative rolls, which means that the characters with more experience in battle are better at acting strategically. Equally importantly, this allows characters who want to do nothing but defend themselves to do just that, regardless of where they are in the initiative order, since focusing on defending oneself is a much faster action than, for example, choosing an opponent and making an attack (full defense is effectively resolved at declaration in d10).

As a player, I've found that d20 gamers critically undervalue knowledge of what the opponent wants to do, when this is in fact the single most important thing to know if you want to make an informed decision about your actions in combat and to maximize your combat effectiveness.
This is best illustrated in a 2v1 fight. Let's say that A, B, and C are in combat, and that A and B are allies. The important cases to be considered are: (1) A wins initiative, followed by B, then C; (2) A wins initiative, followed by C, then B; (3) C wins initiative, followed by A and B.
Case 1 (d20): A acts, completely without knowing what C will do. For all A knows, C could attack, defend himself, breathe fire, or try to use a magical item. A is punished by his high initiative roll in not being able to respond appropriately based on the actions that a less experienced combatant will take. If A defends himself, then obviously C will attack B; C has the advantage in that he's better able to avoid wasting an action, all because he lost initiative. If B defends himself as well because he doesn't know whom C will attack, then C just got a free attack on A or B. Because of this, defending himself is not a good choice to make, so A attacks. B is in the same position as A: if B defends, then C will simply attack A, so B should also attack. If C wanted to defend himself, that's too bad for him, because the actions of A and B have already been resolved, even if C's most natural instincts would have been to defend himself. C can defend himself now, but he won't be attacked, so that will be useless. So C is basically free to perform any damaging action he wants. If C wants to pull out an uzi and blow both A and B away, he is free to do so, even though A and B technically should have been able to respond by trying to interrupt that action... but alas, the less experienced fighter is at a distinct advantage because A and B have already acted. The final result is that everyone attacks everyone else, and the more experienced fighters have less of an advantage BECAUSE they have more experience.
Case 1 (d10): C must declare what he wants to do first, because he lost initiative. This puts A and B at a significant tactical advantage, because the target of C's attack, should C decide to attack, can simply decide to defend himself and have the other one attack C full-on. If C wants to pull out a powerful item, the more experienced combatants can respond by removing C's hand, as they damned well should be able to because they have more battle experience. The result: The more experienced fighters can actually use their combat experience to their advantage and maximize their effectiveness against a less capable fighter.
Case 2 (d20): As explained in Case 1, A should attack. After all, if A defends, C will simply attack B, and that will end up being a free attack for C. If C defends himself, then B will attack him. If C attacks either A or B, B will attack him. The question in C's mind so far is "I'll be attacked regardless - should I deal damage or be useless?" In this case, C should attack. However, if C has a powerful magic item he can use, then B can react to C pulling that magic item out, because B is aware of C's actions before B acts! B makes a more informed decision about his action, a reward he gets for sucking at combat, and so is better able to respond against C than his much more experienced ally. If C fancies a severed hand, he could pull out that item of his and B will promptly remove that hand before he can use it. The best solution for C, then, is to attack. If B wanted to defend himself, it's too bad for him because there is no longer anything to defend himself from, so obviously his best choice by far is to attack. The result: Everyone attacks everyone else, and A, the more experienced fighter out of A and B, is less able to make a tactical response to C than B BECAUSE A is more experienced, and d20 makes B BETTER able to respond to a hostile situation because he has proven that he is LESS able to respond to a hostile situation.
Case 2 (d10): B must declare his action first, then C, then A, so C can respond to any action that B takes before B takes it, and A can respond to any action C takes before he takes it. If B attacks C, C can respond by attacking B in response, attacking A, or defending himself. If B defends himself, C will either attack A or defend himself and B's action will have gone to waste. In this case, though, B knows that A can attack C if C attacks him, meaning that C will be taking two hits, or C can attack A in response to which A will block, which would make C's attack useless. So if B attacks, C has a choice between taking two hits and hitting someone, taking one hit and making a useless action, or keeping his life. If B defends, C has a choice between taking a hit and making a useless action, or keeping his life. So B may want to attack, if he's confident that A will attack. If B is scared and chooses to defend himself, then the fight reduces to a 1-on-1 fight between the two more experienced fighters, as it should. Depending upon how confident C is in his fighting ability, he can either try to take on A while A defends himself and take hits from B as he does so, or he can play it safe and defend himself. The result: A should have been able to take on C by himself if A is a better fighter than C, so obviously with the help of B, even though B is worse than C, the two can put some serious pressure on C. All combatants act strategically.
Case 3 (d20): C can either defend himself from A and B, or he can attack one of them, knowing that neither can defend himself because C acts first. If C won initiative, he could very well be a better fighter than A and B, so it could very well be that C can cripple the stronger of A and B and take the hit from the weaker of those two, but still be in a position to dispatch the other of A and B in a weakened state. If C feels comfortable doing this, he can. Otherwise, this is the only d20 case in which C can adequately defend himself. A and B, however, will both attack C completely regardless of what C chooses to do, because neither of them can defend themselves before C attacks. If C attacked, then what's done is done and A and B should attack. If C defends himself, then A and B should attack. Worse yet, if C wants to use some other item, both A and B have the opportunity to react! C's decision would be a lot easier if he knew what A and B were doing, but he unfortunately does not get the benefits of being a better fighter because the d20 initiative system punishes good fighters and rewards bad ones. The result: Either A and B attack and C defends, or everyone attacks. Worse yet, if either A or B has a powerful magic item tucked away, the more experienced fighter won't be able to respond appropriately, even though he technically has a better understanding of the situation.
Case 3 (d10): C gets to know what A and B are up to before he acts, and A and B, because they are less capable fighters, will not be able to react to C if C decides to pull out some powerful magic item. For A and B, from either perspective, they know that C can attack them if they attack C. If they both defend, then C will get a free pick as to whom he wants to hurt that round. If either wants to use some secret weapon, then C will be able to react to it, so C at least has the two of them in check in this way. If BOTH A and B think that ONE of their attacks is enough to cripple C, then both should attack - C will kill one and be hit by the other. But this is still a bad road to take if A and B don't feel like flipping a coin to decide which of them will die. Depending upon how confident and capable each of A and B are, they should have any combination of attacking and defending. C, the more experienced fighter, can respond to any of these combinations as appropriate, and as he damned well should be able to because he is after all more experienced. And unlike the d20 case 3, C is free to whip out any powerful magic items he has without fear that the less aware combatants can stop him... that round, anyway. The result: This differs from the d20 case in that there is thinking involved, and the situation can go a large number of ways. All combatants act strategically.

Notice that in every d20 instance, not only was the typical solution to attack senselessly because no one knew what was going on, but the combatants who LOST initiative were able to hold the more experienced combatants in check. This simply should not be. Every d10 case was actually required tactical thinking, and the more experienced fighters were better able to keep the less experienced fighters in check.
Not to mention that d20 can muck up some more inventive and/or humorous combat situations, e.g.:
d10: "You win initiative, so the orc declares first. He's going to charge you. What do you do?" "I slam the door." <- amusing situation
d20: "You win initiative. What do you do?" "I slam the door." "The orc runs over to the door and kicks it in." <- amusing situation ruined


RELATIVE POWER

Anything in the d10 system can pose a threat to anything else, and all it needs is a little bit of luck. This gives players hope that they can kill a seemingly more powerful opponent, but also doesn't allow them to completely ignore enemies of considerably lower power. This is good because it doesn't allow the characters to get too cocky, whereas a high-level wizard in the d20 system can think it's humorous to run naked through an encampment of goblins and punch them all out one by one. This shouldn't happen. A geriatric old man who is not trained to fight unarmed should under no circumstances be able to singlehandedly defeat an entire tribe of armed, alert goblins. It's a sign that the game mechanics you're using have power balance issues. "Only a stupid GM would attack high-level characters with goblins" is not an adequate response - the only reason it's a stupid GM in the first place is because it's a stupid idea due to the stupid mechanics. There's a problem when a preposterously large number of goblins have literally zero chance of harming a mid to high-level character.
One thing I've always wondered about the Dungeon Master's Guide is why it even bothers to list poisons. Not only do the characters pretty much become immune to the poisons listed there after level 5 or 6, but most of the listed poisons wouldn't even kill a 0th level commoner child. It might knock 'em out of action for a week or two, but that's really about it. In d10, poison is a serious, serious concern, because 9 out of 10 times it results in painful death, almost regardless of how powerful your character is. The mightiest of kings should still be able to be poisoned, and that's far from the case in d20. The same goes for diseases - has there ever actually been a D&D character who caught one of the non-magical diseases listed in the DMG? Has any player in the US ever had a character catch one of those? And if any did, how many actually died from them, or were meaningfully affected by them?
Upon hitting level 7 or 8, most players no longer care about traps or natural obstacles. Each and every one of the characters is perfectly content to swan dive into 50-foot pits, stroll across spikes, be hit with an arrow trap, or receive a flaming mace to the skull, knowing full well that the damage won't kill them. If the cleric can't just heal them, they have that belt filled with 86 Potions of Cure Serious Wounds. You COULD take three hours to walk back down the mountain from the cliff overlooking the town, but the faster way of getting down is to free fall, faceplant for 10d6 damage (average is only 35 damage), then stand up and walk into town. No big deal. The GM would actually have to override the official rules to sufficiently penalize you for choosing this convenience.


That's pretty much the brunt of it. To me, this is more than enough for me to say that I will probably never GM a d20 game if I can help it, and that I would highly prefer to play in a d10 game than a d20 game, and may not play in a d20 game even if no one around me is playing d10. The d20 game mechanics have proven to be completely twisted, nonsensical, and fly directly in the face of reason. d10 is not only easier to work with, but is also more intuitive, straight-forward, and gives both the players and the GM a much wider allowance for creativity.

-Constantine Perepelitsa, 2007