Short version: Pubic hair, scat, guro, yuri, gritty scenarios.
Elaborated: Body hair and scat I find gross. The presence of blood in any shape or form is a unappealing. It's hard for me to be interested when there's no dicks. Lack of humor in general is a surprisingly big turn-off for me, so naturally I'm a fan of ahegao, silly dialogue, crazy scenarios, unrealistic anatomy, etc.
Other than that there is of course the factor of unappealing art style. This point is hard to put into words. I guess I dislike it when it's realistically drawn, lacking in detail, "flat", or streamlined.