Reddit asked people what may be a deal breaker for them that probably isn't for other people.  For me, I would say a deal breaker would be someone with bad breath. Maybe other people don't mind that too much because there is ways to easily fix it, but uhhh man I can't stand it! Nothing is worse than talking to someone or just being around someone with bad breath, uhh!
Here are some of Reddit's top answers:
1) Littering
2) Someone who is not on time
3) Smoking
4) Chewing with their mouth open
5) Believing in the supernatural
Check out the rest of the list here