Oh...can of worms opened, so time for a list - right?
Well, I'll get started then:
- Canon-Rehash! Yes I know that some canon-happenings might be outside of the protagonist's control especially at first (even if they are a self-insert with prior knowledge), but at some point you need to leave most of canon behind when you've produced too many "splashes" in the pond that is the story! Not to mention that re-telling canon just with say another pairing is just boring! If I want canon, then I'll read/watch/play the original work!
- Slash (not my thing - no, I am not against gay people. Hell, during my last holiday I ended up visiting a street that is well known for gay bars etc.)
- Male-Pregnancy! No, not even with magic or science fiction technology, just no!
- Stupid protagonist not taking obvious openings.
- pregnancy over all (not my thing - don't have kids in RL and I don't want any either...and even worse when I read something like a Harry Potter fanfiction! Damned, they are teens! Them having relationships is ok, as it's what teens do...still, pregnancy at that age sucks, so writing about it in a positive sense weirds me out!)
- Soulmates/Imprinting etc. (Seriously, love is a beautiful thing, but forcing people to be together never is! Hell, we've had that in RL! It was called arranged marriage and the west luckily dumped this decades ago, as it was crappy!)
- "Highschool" (or other RL-Situation) AU! If I read a story, like say about vampire, then taking away the vampirism makes it bland and boring! I don't want the Harry Potter cast in a regular school without magic and Voldemort being a regular criminal or the cast of Twilight not being vampires! Don't get why this is a thing at all!
- inventing siblings for canon characters, as that seldom works well (mostly those siblings turn out to be Mary-Sues!)
- defending the bad guys (unless you change the premise!), if you say write a Harry Potter fanfiction and try to turn Draco into a sympathetic person (he's not, he's a petty bully not worth a shred of sympathy!)
There, done for now - got more, but I'd have to spend a while thinking about it...I think those are enough anyway!