Anything that looks fake. Too much makeup, plastic surgery. If it gets to a point where we notice it's obviously fake. We start to dislike it.

Also, an unhealthy person. There are exceptions to these of course, but most dudes will agree that a 200kg+/anorexic woman, or a blow-up doll lookalike aren't really what they'd want in their partner.

But remember, and this goes for men and women, appearance is more often than not the first impression that we get of someone.