I agree with a lot of what's been said. Unpainted armies, grabbing my models, etc. Can't stand it.
MEQ hate pisses me off a lot. I play chaos, amongst others, and I enjoy them. I also play against a LOT of MEQ armies. But I don't mind. Sure, it's probably more fun to play against different armies, but hey, a game's a game, and I won't deny someone a game just because they happen to like marines. It seems to boil down to nothing more than elitism, and it's really stupid. ("I play Eldar, they require skill and thought, not like your damn fool marines. A monkey could win with them.") In a similar vein, Eldar seem to be becoming the new Marines at my local GW. So I wonder if this will last much longer...
Something else is the sheer anality of some players. I'm all for a good bit of rules debate - hell, I love the R&Q board. But not, for the love of Gork, when I'm playing a game! I'm there to play, not argue over every little finnicky thing. If I'm in a game and a dispute pops up, I'll just D6 for it if it can't be resolved within <1 minute, and then sort it at the end of the game.
Oh, and don't get me started on people who butt into games with 'advice'...