My major annoyances as a GM:
Well, a lot's already been covered, but here's a few that I don't *think* I saw already
-Players that pick up your loose change. Darn it, that's MY penny on the floor. (Ok, it seems petty, but it's still MONEY

)
-Players that invite their non-playing friends/girlfriends/boyfriends to gaming sessions, then proceed to disrupt the game until the GM screams at the top of her lungs that it's unacceptable and threatens the offending party with her d30 of Throwing +4 until they scurry out the door, only to return the next day....
-Players that roleplay too much. Out of game. If you are still your Elven Ranger when you see me at the grocery store, please fail to recognize me.
-Annoying neighbors that want to roleplay, but are horrible at it and have been banned, yet still persist in coming over for various reasons throughout the roleplaying night. Really! <rolls eyes>
-Kids. My major annoyance as a GM. And not anyone else's kids, either - my own. I can't start a game until after 9pm, when they've gone to bed. <sigh>
-The Experts. Ok, I understand if you have a 3rd degree blackbelt in 90 different martial arts that you know what you're talking about when a certain move is made. I don't. Details are fine on a need to know basis, but when no one cares, or needs to know the difference between move x and move y and how move x is more appropriate to the situation, yada yada, please refrain from the 30 minute tangents.
-The Phony. The long lost cousin of the Expert. You know the one. He says he's got a 3rd degree blackbelt in 90 different martial arts (and he *may* think it's true), but he doesn't have even a white belt in anything. The one that, when he finds out you're a pagan, is all of a sudden a Warlock of some dark, sinister powers....
Ok, I think that's some of what I've had to deal with...of course, all the really good ones are already mentioned
