for me, right now, most anything is a dealbreaker.
Tolerance feels pretty low.
granted, I'm coming out of a relationship, and it was too much of a waste of time, so I'm kind of bitter
Anyways, aside from the big obvious ones (money, cheating, abuse, trust, respect) ..
I would say
compatibility.
because, how one is, in terms of what they like, how they function, on a more superficial level, make up who they are.
neither partner should expect that to change.
how one is, in terms of interpersonal skills, provided it's not one of the Big Ones noted above, that can be worked on.
In other words, a person can and should always try to be a better communicator and so on ... but if someone likes the color pink and is pretty set in their taste in music and movies and is extremely independent, .... that may be just how they are, and that may not work in the long run.
mis-match, could-be-better-match.
whatever. go fishing.
I've learned, it takes a lot more than love to glue a relationship together.