I prefer a balance of roleplay and combat. Too much of the former and it stops feeling like a game and becomes awkward and boring. Too much of the latter and it becomes a shallow grindfest. I tend towards playing the game fairly seriously (though I inject a little comic relief into all my characters, and am up for pure comedy campaigns too) and try not to spend a lot of time screwing around chattering. I like to keep things moving. I also tend to put a LOT of thought into my characters, both mechanically and story-wise, though this varies with campaigns. My characters are always designed to be able to contribute.
I WILL NOT join a group of over 6 players. 6 is the limit of my patience, and I prefer fewer players than that. I also don't do PVP. At all. I hate inner party conflict and will quit a game sooner than fight another player. Other things I don't like/ don't do include metagaming, optimizing (different from making informed choices to make a solid character. i use this to mean making something deliberately game-breaking), and intentional plot derailment.