Put me down for a second "character advancement is only tangentially related to XP" opinion.
I mean sure, there are some very rare examples where a character requires some benefit he gains from levelling up in order to represent some aspect of his character that isn't linear progression of power (ie - you may have someone learn to use an item that he's found or a new language. The current 4e system says he can only possibly do that by gaining a level) but in general the only thing that levelling up (and the reciprocal levelling of monsters) does is to give a feeling that new threats are bigger and badder, and to keep giving players new and interesting things to do.
The only time this isn't the case is if you're one of those 'the world and all the monsters in it are set in stone ahead of time, so you'd better level up if you want to go to the place marked "here be dragons"' DMs (not that there's anything wrong with it, it just seems like a lot of extra work to me). And even then the XP curve is really behaving as a sort of drawn path through the world, not any kind of realistic gauge of characters changing and improving due to the world around them.
That also partly why I don't believe in any sort of differential XP system in D&D. 4e (and D&D in general) breaks down if one character is significantly ahead of the others. Differential XP just makes the DMs job much harder. Since there are plenty of other things to give out (magic items, boons, rerolls, roleplaying rewards like your own castle, RL rewards like sweets) that are much more fun for all involved and much less work for a DM, why would you choose XP?