The deal breaker; that one thing (or perhaps a couple things) that you absolutely cannot overlook, that makes it so a relationship (whether romantic, platonic, or even professional) cannot work out.
Do you have one? And if you do, what is it?
For me, the biggest one is obviously deception; if I can't trust a person due to them lying or deceiving, then that relationship is doomed to fail. One or two lies might be able to be overlooked, but if it's pathological lying... then it's obvious that trust can never be fully established. It applies to all types of relationships.
However, romantic relationships have a second deal breaker for me... smoking. As an asthmatic, just being around smoking makes me cough and wheeze, and can trigger an attack. Not to mention it smells horrible, so I couldn't imagine kissing someone with smoke-induced death breath. I could never make it work with a smoker...
So what is your deal breaker?
Edit: If this has been done, mods, feel free to close it. I looked and didn't see one, but I may have missed it.