Barak said:
The reason rules-light systems work well for roleplaying, IMHO, is that they come packaged with a different socail contract.
If we're again using "roleplaying" to mean "immersion", I'm not sure that I buy the first part of this statement, as I have not seen it true, across the board, in my play experience. That is, I haven't seen "lite" be especially better or worse in this regard.
As for the second part, I can't think of many games I've read, lite or no, that address social contract directly at all (which is part of the problem with many RPGs). Ergo, I'm not sure what social contract is being "packaged" with them. Of the two I've read that do address contract fairly specifically—
Burning Wheel and
Dogs in the Vineyard—the former is quite crunchy and the latter pretty simple.
I've also seen both heavy and "lite" systems that do the bait-and-switch tactic of presenting the GM and players with mutually incomapitble guidance, which I would consider an impediment to "roleplaying" (both "immersion"-wise and in general).
I think it comes down to what mhacdebhandia was getting at, i.e., whether your goals are compatible with what's facilitated by the system. If the system gives absolutely no incentive to play a one-legged wizard with an Int of 8, then I can't blame any player for not wanitng to play one, and I'm not going to call them a "minmaxer" when they choose a more effective PC.
If I was really after making that dull, one-legged wizard an interesting RP'ing experience, I'd look for a system that rewarded those choices. E.g., one with an ads/disads system that gave me extra points for being one-legged and not-so-smart, or one that allowed interesting things to happen whenever those complications arose in play.
So, in that sense, system certainly can get in the way, but not so much (IMO) w/r/t complexity, but in whether it facilitates the kind of play you're after. I.e., "System Does Matter".