Due to a discussion taking place on a messageboard that I frequent, I've given some thought to the desirability of "personality" mechanics, and other tools that exist to facilitate roleplaying in RPGs. Some of these mechanics have been floating around in various systems for a number of years. GURPS certainly popularized a large portion of it with its traits and flaws mechanics. The so-called Storyteller system, which billed itself as a system designed specifically for roleplaying (as opposed to merely combat and miniatures gaming) also heavily used them (by the way, I could make a whole 'nother post on whether or not Storyteller really met that goal, or if they merely wrapped that like a pretentious Christmas wrapping paper around a system that "on the ground" didn't encourage any particularly unusual or innovative behavior in its players than any other system ever did.) D&D itself has toyed with them; Andy Collins' Third Edition version of Unearthed Arcana had both traits and flaws; traits being trade-offs; you get better at one thing while getting worse at something else, and flaws being merely a way to get worse at something; but to get a free feat out of the deal.
Anyway, the guy I was chatting with is very much in favor of this type of mechanic, but the more I think about it, the more I don't really care for it. I've discovered over the years that I don't care a whole lot about mechanics. I bought into d20 in 2000 and when the d20 rendition of Call of Cthulhu was actually a very good game (and diametrically opposed thematically to D&D, Star Wars, or other offerings we'd had so far) I threw in my chips and counted myself a d20 player.
Although there are still things about the system that I don't like, I've made my peace with them and accepted that the Holy Grail system that's absolutely perfect for everything I could ever want doesn't actually exist, and I'm not even sure what it would look like anyway. The other advantages to this is that d20 is an incredibly robust system. If you need rules for something, you've probably got it out there somewhere, and it's pretty modular in how it can be used. I suppose if you slavishly adhere to ever detail of the rules, you'll find yourself quickly in over your head, but I've always seen d20 as a toolkit (remember the saying that Wizards of the Coast used to throw around in the early days of Third Edition? "Tools not rules?") and the rules as such do not trump GM adjudication. I don't need to know every detail of every facet of using, for example, the Jump skill. If I have a PC that wants to jump, I whip up a DC on the fly that seems reasonable for what he's attempting to do, and tell him to roll it. So, for me, the robustness of the d20 rules isn't a tedious hindrance, but rather a full toolkit that I can dig into to find what I need, and leave what I don't sitting there unused until I do need it.
And d20 doesn't really have any roleplaying mechanics per se. The character options are really (mostly) more about combat capabilities then they are about your actual character and personality. Alignment, I guess, passes muster, and the Traits and Flaws mentioned in Unearthed Arcana do too. My experience with using them (mostly in games other then d20, but mechanically they were more or less identical) is that roleplaying mechanics do not actually promote better roleplaying amongst players who are unlikely to be good roleplayers in the first place, and in fact lead to problems with an extremely annoying breed of metagaming and mechanical abuse.
Good roleplaying isn't something that can be mechanically enforced, and I'm skeptical of how well it can even be mechanically encouraged. If you want good roleplaying, you need to find a group of gamers who value it for its own sake and do it on their own because that's how they play, not because the system does or doesn't reward and/or penalize them for attempting it.