So for me, there's obviously the aforementioned
Vegans: Meat is love, meat is life, nevermind all the other animal products they refuse eating. Foolish, stupid and just sucking the fun out of life. I just couldn't live with somebody like that.
Facial Piercings/Body Modifications: I just can't stand them. If they're elsewhere, I could deal with them, and earrings or clips don't really bother me. But stuff up your nose, brows, lips or tongue? No thank you. Also keep your earlobes intact, thanks. Most of these things might be solved by taking the piercings out, of course, but eh.
Drugheads / Smokers: Just no. If you have a history with trying drugs, that's one thing. But if that's still ongoing, that'd be a dealbreaker. I have friends who occassionally like to get high (I don't), and can tolerate it to a degree, but I wouldn't like to be in a relationship with a druggy. Even smoking is a big problem that'd be a reason for me to turn around from that sort of thing.
Long-ass fingernails, especially painted ones: Get the fuck away from me, Freddy Krueger. They ain't nice, they're a safety hazard. Brrrr.
Now, the floor is yours.