I guess mine are just the basics.
Any physical abuse. Red line. No second chance.
Emotional abuse, I'd probably leave. But that's more insidious, slowly creeping into the relationship. So difficult to pinpoint where the tipping point would be.
Cheating. I'd leave.
I read something once about how important respect was in relationships. Once one partner no longer respects the other the relationship becomes unviable. And that struck a chord with me. Dh really listens to me, my opinion matters, he always treats me with respect, would never belittle me, etc. He does sweet kind things, little things like a cup of coffee in bed every morning.
My relationship prior to dh my boyfriend lied. A lot. About stupid stuff. That he really had no need to lie about. But I found I lost all respect for him. I couldn't trust him, couldn't believe him, and although it really broke my heart at the time I broke up with him.
But I'm in a fortunate position where I'm financially independent, I have enough self assurance to know my worth, etc, so splitting up would be easier than it would be for others.