This is all hearsay since I've never been in a relationship, thus wouldn't really know what I wanted.
They're probably all petty, but here goes...
Not being intelligent.
Arrogance
Ignorance
Critical of the stupid things that make me happy. Obv they make me happy; stop pissing on my parade
Being too close too fast
Selfishness.
Being too serious. I'm only pretend serious.
Homophobia would not go over well.
Racism.
Overly religious.

Not wanting kids would kill me if it were a serious relationship. So would not believing in marriage. I'm old-fashioned in some senses and would want him to want those things too.