Scat. Watersports. Necrophilia. Guro. Beastiality. Pedophilia. (Last three don't really apply to this site, thankfully.) Feederism and some of the associated weight gain/slob sub-genres are also not anything I'm keen on.
Sexual violence has never been something I'm into, nor is pain-for-pain's-sake. BDSM within reasonable limits and generally consenting parties is fine, and non-consent has its place in sexual fantasy, but I've never admired straight-up rape fic. Even with mind control, I prefer influence to straight up "I give him/her their orders" - that's more of a power fantasy than a sex fantasy to me, and while plenty of folks want that, it's not generally what I'm looking for.
I dislike bad anatomy (i.e. normal humans with impossibly-sized genitalia, impossible insertions, etc.), but I have a soft spot for impossible anatomy when in the service to fantasy (surgery, aliens, magic, super science, etc.), and when dealt with realistically within the context - i.e. yes, Superman could plausibly use Plastic-Man as a condom; no, Wonder Woman could not fit a normal-sized fire hydrant up her vagina...unless magic or something was involved. Even then, it's about establishing the setup for the fantasy as well as carrying it off.
Bimbofication, hucow, and related very specific combinations of physical and mental transformation leave you with a character that isn't recognizably like the one you started with... so it ends up being very depersonalizing, unless handled carefully. A long slow slide into degeneration or prostitution is more interesting to me, especially if the individual has opportunities to make their own choices, fight back, say no in a meaningful way, give in to their desires, take control of their sexuality, etc. The "spiral of despair" isn't exactly something I'm keen on, but I do like the "discovering a new side of myself." Still, it takes a bit of skill and effort to pull off well. Magical personality/body re-writes can be an interesting premise, but get tiresome if addressed briefly and in quick succession like some of the superstories threads. Magically quick transformations work, but only if you take the time to then emphasize the effects and consequences.
Racism. Mostly you see this in some interracial stories, especially "black breeding" stories. It's a taboo that Crossing the Divide and others play with while carefully avoiding going over the line; I think it's appropriate to stop short of naming any race as superior/inferior, or talk about "pure white heritage" yadda yadda. Just not my cup of tea.
That's not trying to kinkshame anybody - hell, read the crap I write - just personal preferences, and why. Everybody is apt to feel different. I might not be into cakesitting or diapers or adult babies either, but that's not because there's anything inherently wrong with those.