Quote Originally Posted by Firechanter View Post
Although I have to say, fiddling with character concepts and trying to find ever new ways of kicking ass is half the fun. And that's kind of what sets D&D 3.X apart from anything else (at least the mainstream stuff) -- there are thousands and thousands of options, classes, feats, spells, you name it. This extreme wealth in character options. And it's really the reason why 3.5, despite all its shortcomings, is still my favourite system.
Basically, this. If you're looking for a reason to play "D&D or any other system", then your reasons vary widely. If you're looking for a reason to play "D&D specifically", then your rational reasons narrow to something like: optimization, popularity/familiarity, ease of using setting-specific fluff. (There might be a few others but those are the main ones I can think of.)

So one of the biggest differentiators for D&D is optimization, though that doesn't define as much of the total play experience for someone new to RPGs as a whole.