Honestly, I'm a pretty forgiving person which has left me in some really blah situations relationship wise. -__-
Now...if I did have some kind of backbone so I'd be able to leave my SO when they did something I didn't like, these would probably be qualifiers:
-Any form of abuse
-Manipulative behavior (tho I suppose that fits in with the first point)
-Cheating
-Frequent dishonesty
...and just, not feeling happy with the person anymore. Like, I was in a relationship for a few months, and they basically made me feel constantly guilty and like I was always screwing up, and would snap and get mad at me for no reason. I wouldn't want anyone to have to be in a relationship were they're feeling bad all the time.