I play D&D because I like the "feel" of D&D. And contrary to a commonly-held sentiment, I'm of the opinion that the system makes a major difference in the feel and style of your game. A few key things I like about D&D as a system include:
- Vancian magic
- Class-based PCs with levels (the more this gets diluted by skill systems, the less like D&D it feels, to me). I like strong archetypes with clearly-defined roles.
- Roll to hit with a d20 vs. AC, roll damage vs. hit points
When you start getting away from those elements, I start losing that D&D feeling.
Other things I like are swords-n-sorcery elements: lost cities covered in steaming jungle, psionic ape-men living in caves and worshipping Great Old Ones, foul sorcerers in league with demons, giant frogs, jeweled cities that are the pinnacle of civilization and also treacherous dens of iniquity, long-buried alien artifacts that wreak havoc when they're brought to light, et cetera. Those aren't so system-dependent, of course, but D&D works well when modelling this kind of swords-and-sorcery feel, IMO.
I find that D&D (and especially OD&D or AD&D) gives me the kind of game I like. To the degree that 3E had started diluting the system-elements I prefer, it felt less like D&D, to me. That's one reason I drifted away from running 3E; it didn't fit my game as well. Everything I've read, so far, indicates that 4E will continue to widen the gulf between the current edition and my preferences. So I run what I like.