I run a group with serious, heavy roleplaying, but at the same time, smash a barmug in the face, kick in the junk combat.
For the most part, I've found that, like anything else, the rules don't inhibit roleplaying or encourage it.
We don't let the rules make the game. They serve the game. This thread seems a lot like the "Do the rules serve the game?" thread, where people are wondering if the rules should be the end all be all of all the game.
In there, I pointed out the habit that seems to be growing that all non-mechanic prerequisites for feats & PrC's and templates and everything else should be removed.
If roleplaying is so important in gaming, than why are so many people clamoring for the removal of those non-mechanics from the game? Why do you hear the battlecry of "IT'S NOT FAIR!!!" whenever the players find out that thier GM won't let them be half-dragon, half-dragon, half-golem, half celestial Sorcerer Rune Mage Barbarian True Necromancer Red Wizard Blackguards?
"But I make all the prerequisites, and everything is supposed to be for the players as well as the GM!" is heard.
I think roleplaying is subtly discouraged because the players can fully audit the GM, the game world, and a lot of the mystery is gone? Before, if the mage tower had a cool floating section on a cloud, it was just accepted, now, there had to be mechanics behind it, it has a certian DC forumulae for dispelling, and even what colour it can be is regulated.
See, it's not the roleplaying that you guys are actually missing out on, it's the over-regulation on everything has a tendency to stifle everything, allowing some to gain what they finally wanted: A game that dances to thier tune with no mystery, nothing unknown, a complete lack of mystery and inherent magic within it.