ooh, I've got a couple.
I'm a high school English teacher, so I get annoyed with homophone usage in posts i.e. "there" when the poster means "their", stuff like that, as well as really atrocious misspellings (typos I can understand)
Long posts with no line breaks or punctuation. It makes you seem like the Unabomber or something.
When people revive a really old thread (several months, say) with a question that has nothing to do with the original thread, and could have just as easily been a new thread.
And the big one, the fact that I don't get out enough, and instead spend time writing about my pet peeves online.