For me, I would think stuff like
-Insecurity. If the other person is so insecure about themselves or thinking you cheating on them, thinking they aren't worthy of you, if they can't address this with you, or if you can't be honest and tell them that the constant negativity is hurting the relationship, you shouldn't be with that person
-Hygiene. If they have bad breath or questionable odor, you should be able to talk about that with them
-Money management. If you think they are spending too much money on dumb shit, let em know.
-Relationship with family. If either or you don't like the other family, or they don't like you, that should be something you should discuss with them. I know someone wouldn't want to cause all divide, but still
-Carrying too much of the load financially, and in terms of doing stuff around the house. Also if one feels drives too much and feels it is annoying, they should encourage their significant other to drive, or at least let them drive you around for a little until they get their own