Alright, I'll give this a shot. Heres a bit cautionary note however: I'm an asshole, like goatse size.
I have only one absolute, no discussion deal-breaker and thats cheating. Been there, it sucked, not interested in ever being there again.
A list of minor ones that at least involve discussion before action is taken:
-Someone becoming dependent
-Repeated poor, life decisions
-Me becoming their 'life;' i.e., their friends disappear, they have nothing to talk about that doesn't involve something done together, etc.
-Lack of sense of humor (understanding that everyone has their no joke topics)
-Trying to change me, particularly trying to convert me; this also covers friends and the like
-Rampant, emotionalism
All that said, I'm big about calm, adult discussion. I can forgive most things so long as its been made clear that I'm not comfortable or was hurt by whatever and the other party recognizes and respects that. I'm also aware that no one is perfect, mistakes are made, by myself included, so intention tends to be a big decider on what is forgivable. If someone is intentionally being malicious, then all bets are off.