As a bicycle commuter, there are a few things that drive me nuts. People who zoom out of parking lots/driveways without looking. Motorists who shout "get on the sidewalk". But when it comes to other cyclists, I really, really really hate when they zip past me when I'm stopping lawfully for a traffic signal or stop sign.
Okay, okay sometimes my stopping has more to do with a strong desire to catch my breath (say at the top of a hill) than really /wanting/ to be a law-abiding citizen, but when I sacrifice my momentum at the base of a hill for a traffic signal, it really gets my goat when some other dude just... doesn't. Does he think the traffic will just stop for him? It's rude, illegal, and very, very goat-getting.
My number two peeve is being passed, in general.