For me, it's people who walk really slow (especially down a narrow street where you can't overtake them). It doesn't annoy most people, but it really gets to me. I don't know why, but I find myself grinding my teeth and grunting.
Another thing that annoys me is when people stand somewhere you need to go, talking. For example, I was in McDonalds today and two people were standing, talking, in front of the place where you get straws. I got really angry, inexplicably.
When someone has their pants pulled down to the point you can ask why they wear pants in the first place. In high school, it pisses me off when I'm stuck behind someone who does this walking up the stairs and even worse when they do it with a belt on. Do not mistake me for an old guy asking to pull up your britches please.
Another is people who are close-minded and are unable to open their minds to anything except for what they believe in without even considering something else. *cough* RELIGION *cough*
Another is people who make generalizations about a broad group of people, without regard to their vast differences *cough*C++ PROGRAMMERS*cough*. It's like saying people are idiots, while sometimes true, I could show you someone very intelligent.
Strangers that meet you in a building and say good morning to you or whatever and reiterate with more emphasis if you don't respond, as if demanding to be greeted back. I'm not talking about a janitor or someone like that. I'm talking about someone who just happens to use the same building but is otherwise completely unrelated to you.
I swear, if someone does that to me again, they'll find a surplus of my knuckles in their face.
I know this makes me sound rude, but I don't even know these people. I'm not more related to them than to someone I see on the street.
I know what you mean. Another is people who smile at me in the street that I don't know. Who the hell are they? I don't know them! So I turn my head the other way.
Well, you can do that... It's just a bit creepy. He had a visible pistol hanging out of the back of his pants, looking at a window, then he turned and walked behind me for like, a minute. When I turned, he fortunately didn't turn or I would've started to run T.T.
EDIT: Edited the part that made it look like a cheesy gay porn.
I know a man (this is for real, BTW) who spent some considerable time in Japan.
While there, he came upon an old fellow in a wheelchair desperately trying to get up a ramp and into a store. He couldn't, because in order to operate the door he had to release one of his wheels, so he was unable to get in no matter how hard he tried.
During this time, there was a near constant stream of people coming and going (in Japanese that is just one word: iku) through the door. The old man was in the way, so they would have to squeeze around him.
Finally my friend ran over and held the door for the old man so that he could wheel through into the building. The old man was flabbergasted that someone would stop and help!
Japanese people aren't evil. Their culture just has them avoid at all costs people they don't know.
So the next time someone has the intelligence and social grace to smile at you as you go on your way, perhaps you ought to consider moving to Japan. Or hermitage. Either will do, so that kind people don't have to look at your sour face.
Of course, those who lack the social grace to respond intelligently to a "hello", or to the lack of a response... well, they deserve each other --as often as possible.
Oh, wait. I'm talking to a bunch of programmers. They wouldn't know the difference between a comb and deoderant if it hit them in the face.