1) People who refuse to accept someone's word about what their religious beliefs are. A person's religious beliefs are one of the most personal things they have, and it's not up to someone else to decide if their concept of God is "pure" enough to be able to claim membership in a given broad religious classification.
2) People who try to take citizenship away from American citizens. I'm OK with people who want to change the Constitution to affect people not yet born, even if I wouldn't support their proposed change; that's part of the traditional political process. It's the people who want to retroactively change the rules to take away citizenship from those who already have it that piss me off.
3) "Africa" by Toto. I know it's an old song, but I'm still pissed off that a song with insipid lyrics like "I know that I must do what's right, as sure as Kilimanjaro rises like Olympus above the Serengeti" could ever be popular.
What's pissing you off nowadays?