I've always thought about roles. In every game.
You want to make sure not all the PCs are the same, and don't stomp all over one another's niches. So you make them different from one another -- one's a ranged dude, one does magic, maybe one runs really fast, whatever. Maybe one has a stronger psyche, one is physiclaly the strongest, one's got the best warfare, one can take the most punishment, and one's pretty good at all of these, but also has a bunch of dirty tricks. Or, to look at our Feng Shui game, we've got one defender-type (the teenage martial artist with the Scrappy Kid defense and a horde of martial arts), one "take down multiple enemies" controller-type Sorcerer, and one "buff, debuff, and combine sorcery and martial arts for one big attack" striker/leader Taoist.
Basically, as long as the PCs are sufficiently different, you'll always have roles. What those roles actually are will depend on the situation and the system -- but while D&D has focused on the roles to make sure every class is useful, it's not those roles that matter in play -- it's whatever setup works best to deal with whatever threat you're currently facing with whatever team you have now. Maybe that means having someone play bait; maybe it involves catching the foe between the striker "hammer" and the defender "anvil"; maybe it involves the wizard pinch-hitting into striker or leader mode against a single foe that's not effected much by controller-esque powers. The roles you use in play have -some- resemblence to those used in design, but really, you do what keeps your character effective, distinct, and interesting.