I'm with you on the AU thing, Mare. I don't even like when they're brothers instead of friends. When I write AU, I write the Boys as if they are the same people, but made different life choices - instead of being part of a boyband, they did something else. So I like to read AUs that write them the same way. It's different if you're writing "original fiction" with characters that may be based on the Boys, but if you're labeling it fanfic, I think you should keep them as true to life as possible.
My pet peeves are mostly related to grammar, spelling, punctuation, and English conventions in general. There are plenty of stories out there that are probably well-written and just aren't something I'm interested in, but the ones that are not even as well-written as the stories my 9-year-old students write annoy me... mostly because they make us teachers look bad. LOL Why won't you apply what your teachers tried to teach you???