Most of mine have been mentioned already, but here they are anyway.
Smoking/drugs/alcoholism. I'm allergic to cigarette smoke, hate the smell of marijuana, and just plain don't like the effect that unmoderated drug use has on people. I'm okay with occasional drinking, but that's about it.
Depression/neediness. I can't handle the emotional strain that comes with being in a relationship with someone who's unhappy all the time. I'm also not emotionally expressive by any means, so I'd have a hard time dealing with someone who always looks to me for explicit validation and support (not that I wouldn't be supportive, I just don't like being forced to vocalize it.)
Fighting. I don't like negativity, so someone who makes a big deal out of small situations out of boredom, or starts arguments to try and connect with me just isn't for me.
Power dynamics. I'd want my relationship to be simple and genuine - I'm not interested in the more intense side of romance. One-upmanship in flirting is bothersome, and turning dating into the type of seduction tug-of-war that pop culture often depicts just sounds exhausting. I don't see the appeal of being dominant or submissive: mutual respect between equals is what I want.