About the only thing that really creeps me out, instead of just going "ick, not for me",is revenge porn. You know, all those fics that seem to basically exist so that someone who has an axe to grind with a character can swing it at said character until it's nothing but mush. There's a level of sheer hate involved in so many of them that rather puts me off.

Well, that and the ever famous romanticizing of horribly abusive relationships. That also rates pretty highly in the creepy-meter.

But you find both of these in published stories, and plenty of them. They're not really just fanfiction trends.