People who blare their headphones on the bus. Personally, I don't care, I can easily tune it out; but how can someone be so inconsiderate of other people? The bus driver, especially, is affected by it. They need to concentrate so they don't drive the several-ton bus off a bridge and kill everyone because someone wants everyone to know what kind of music they listen to, and that they don't give a shit about anyone else.
Last night, on the way back from a class, some douchebag was playing a radio of some kind. It wasn't from headphones, it was literally a coming from a speaker meant for hearing from a distance, and he was just playing it like he was in his own fucking room.
In conclusion, people who don't consider other people's feelings are what peel my potatoes with paring knives of anger.