I really doubt if lord_day needs an introduction. If you don't know who lord_day is, scroll up. See those pretty numbered links with the pound sign at the beginning at the top of the page? Click on pretty much any one of those and you'll find a map by this guy. There's some important stuff in this interview. It's pretty rock. Outside.

Tanner: You make a lot of maps and participate in an almost unparalleled number of map packs. Do you ever worry that you'll simply run out of ideas someday?
lord_day: Sometimes I get that fear, but not too often.  There are so many unexplored ideas for levels, and so when I do run out of ideas I just make maps that are interesting in some way or another, be it a new look or a new object combination. Also, playing through large numbers of maps helps me think.  Sometimes I will see what an author hasn't done in a map, and make my map out of what is missing in their map.

Tanner: You've been in nine map packs so far. Which has been your favourite map pack from the ones you've participated in?
lord_day: Oh boy, that is a tough question.  I think my favourite map to participate in was Session 4, which was just me and rocket_thumped, trading maps and working real close together.  We were a real team, and the time spent making that pack was awesome.  However, my pack with which I am most pleased with is my solo pack, the completed column.  Making 50 levels by yourself is extremely satisfying, and I explored many different aspects of map making in that pack.  I really evolved as a map maker through it.

Tanner: When you make a map, do you just sit down and say, "I'm going to make a map." or does there have to be inspiration?
lord_day: When I sit down to make a map, I always have something planned out in my head, I don't like to stumble blindly into NED and just see what comes out.  Perhaps that's why my ten minute maps aren't great. However the degree I have it planned out to depends.  Sometimes I have the whole map in my mind, and sometimes I just have a little area planned out, and it evolves from there. I also think that's why I'm not very good at using pre-made tilesets.  I like to place my objects and tiles together, so that I have full control of how it all works.

Tanner: Do you ever completely abandon maps?
lord_day: I do abandon quite a lot of maps, but it's rare for me to abandon a map that I finish.  Usually if I am going to abandon it, I can tell before it is half complete.

Tanner: What are three things you always keep in the back of your mind while you are mapping?
lord_day: I'd say number one, is that it's enjoyable to play.  I think most map makers keep that in mind, however some people sometimes are so concerned with making aesthetically pleasing maps it clouds this idea.

Number two would be that I need keep thinking critically about my maps.  I ask myself 'Is that idea really cool?' or 'Is that actually fun to play?'  It's easy to fool yourself into thinking that your map is really good, when it's not.  I've play tested a lot of maps, and most of the time the bit I think is the worst is what the author thinks is the best bit of their map, and I always feel that they are pulling the wool over their own
lord_day:

Number three is that it looks good.  This may seem like it's contradicted number one, but it's not.  A map can be fun to play, and look beautiful; there is nothing better than playing a fun, good looking map.  A good looking map subconsciously signals to the player that the author knows what he is doing, and it makes them feel that they are in safer hands.

Tanner: What real-world skills does it take to be a good map maker?
lord_day: I think there are two obvious things that help.  Firstly, as I said before, being able to think critically about your work is important.  Looking down as a third person can really help you in real life as well as NED.  I also think that if you want to get your name on the map, you need perseverance.  You will not become known as a good map maker over night, and I'm sure many map makers had to just keep on trucking through NUMA until they were recognised.