The use flag though powerful is quite a pain for the end user when he does not know the additional support he requires.  A very good example is vlc where if proper use flags (which are tons in number), if not used properly the resulting compilation is hardly usable. I propose on building a tool which will use XML data provided by package authors / or other users which will allow the user to configure his installation (i.e. choose use flags) a lot easier. Similar to an installation system where the user can choose the features of the software he wants to be installed.