I was having a discussion recently with Ben, a great friend and member of my local group here in Chicago, about the merits of gaming systems vs. gaming groups. Lately Ben's been on a ruleset kick; he's been buying PDFs of various games and monkeying around with different mechanics. I guess you could view this as something of a research project, since Ben is set on writing his own full-length RPG in the next year or two.
Anyway, we were discussing the recent rash of indie games that take narrative control out of the GM's hands and distribute it amongst the players. Games like Burning Wheel (whichn I'm currently playing) encourage players to base dice rolls around their more esoteric traits, rather than waiting for the GM to instruct them to. (Ex: Using "Nobleman's Son" to defuse a potential combat scenario rather than Persuasion or simple fighting. There's no stat attached to it, but the GM can react appropriately when the player asserts the trait.) This drives players to resolve encounters in a variety of ways, both social and combative.
Ben is really jazzed about systems that provide for this sort of character-driven interplay. I, on the other hand, have been playing devil's advocate a little and suggesting that the right gaming group doesn't need to be poked and prodded by a quirky ruleset. Roleplaying will just happen, without any sort of conditional system to encourage or refine it.
Case in point: the d20 system is the yardstick for measuring most new RPGs. It's fairly traditional, somewhat versatile and satisfies most rule-hungry gamers. It doesn't really promote roleplaying through written mechanics - but that hasn't been a problem in my D&D campaign. I'm playing Midnight, a fantastically dark setting by Jeff Barber, with a group of guys here in Chicago. We're all about the same age (late 20s to early 30s) and we're all remarkably on the same page as far as what we want out of our game.
As such, we don't have any problem engaging in some nice meaty roleplaying within the parameters of the d20 system. I'd point to this as evidence that the game group, not the system, ultimately determines how satisfied everyone is with the RPG experience.