I despise seeing playbills thrown on the floor and left in the theatre after the show. It just makes me sad that people don't care enough to want to keep them.
Late comers! Gah! It's not so much that they are late, it's when they're sitting by me and my view is abstructed by the people in their row trying to accomodate them. It's very rude.
Rude ushers! They have no business being so rude.
Outside food in the theatre. During "The Pillowman", the woman sitting next to me popped open a can of soda in the midst of the play. It was rather loud, and we were sitting close. She also didn't know who Jeff Goldblum was, and had a comment to make about every actor in the show.
People trying to explain a joke to their friends. During "A Naked Girl on the Appian Way", I sat beside a couple who were less than thrilled to be seeing the play. The man did not get any of the jokes, and his wife (loudly) explained them all to him. I also hate when audience members yell back to the stage or clap along to the music.
Singing along is a major pet peeve of mine. I sat in the front mezz of "Rent" and was distracted by a girl in the front row singing along to every song. Every time I looked down, I saw her.
A little swash, a bit of buckle - you'll love it more than bread.