Oh, that's easy: incest themes, including all the "but they're not related by blood so it's different and I'm in denial about what it means to have an adopted sibling" incest. I don't think it's a healthy fetish, and I don't care that it's out of vogue to call any fetish involving consenting teens or consenting adults unhealthy around here.
It's worst when I've been enjoying a new anime when suddenly Surprise! It's also about incest! When the anime's good enough and the incest is secondary to the main themes I don't think it's necessarily a deal breaker, but it never helps and it drastically limits the kinds of people I can recommend it to. I still love Garden of Sinners but man would I prefer it if Mikiya's younger sister didn't want to get in his pants. It helps that the characters in GoS are supposed to be kind of sick, but that doesn't change the matter that they're trying to appeal to a fetish.
tl;dr: incest fetishism is bad and it limits the potential audience for anime in a bad way.
