Some of mine have already been said, but they can't be stressed enough.
The sentence-level errors in some fan fics are horrible. It is beyond me that people will post, wanting others to like the story and review, but not bother to look over the chapter first. Editing is essential to being a writer. You're not going to catch every single mistake, but when you don't try? :'(
Which leads in to my next pet peeve: when you give someone constructive criticism on a certain type of error that keeps occuring, and the author doesn't bother to fix it.
As far as characterizati
on, I get annoyed when the leading female in a story is completely cliche and undeveloped. There is absolutely nothing unique about her, yet one (or more) of the Backstreet Boys magically falls in love with her for no apparent reason. Hmm....