People who think that their alignment is a suggestion. Case in point: I have this guy who has LG on his sheet, but he was consistently a dick to this one guy whose character was childish, and yet somehow managed to attract a cohort who was a child, who coincidentally happened to hate the other guy's character on principal alone (she was half-drow, him an elf). I repeatedly told him that doing that was not very good, especially since he's lawful good, meaning that he has to be good to everyone. At the very least change your alignment to neutral, or lawful neutral, or something else that makes fucking sense.
People who think that their alignment is an excuse for dickery. Case in point: Same guy, who like to play half-giants, made a chaotic neutral character who only spoke to ask when he got his money. He was also generally a dick to everyone, and then he apologized for it because he said "that's how my character would play it." Dude, what the cunt? If you don't like being a dick, don't make characters that are megadicks!
People who give freebies to other people. Case in point: Same guy gets sexual favors from the DM in the form of free leadership, exotic weapon proficiencies, armor (even though it specifically says that fullplate costs x amount of gold to get re-fitted), etc.
Leadership. It's not balanced, it's not fair, it's too good, get it out of my games.
Games where everyone has leadership and/or special mount and/or animal companion. It's ridiculous. I need the animal companion just to stay competitive in the action exchange, and I'm a goddamn druid!
People who bitch. On and on. About you not accepting their characters for which there are no rules governing the things that they want. For instance, there is no CR exchange rate, and an animal companion cannot (as far as I know), acquire a level adjustment.
People who bitch. On and on. About how the system that you use, like Tome of Battle, is so unbalanced, and that a fighter cannot do all those things, therefore it's too strong. And they use that "holier-than-thou" voice, and pretend to be cool with it, and refuse to bend even when confronted with hard numbers and personal experience.
How only 10% of all spells are worth casting because the others suck so hard and the 10% that are worth casting are completely gamebreaking and should never be allowed ever. Ever.