These are my deal-breakers, if the significant other:

~ has an STD
~ is a radically, overbearing religious freak (although I'd like for him to believe in God and Jesus)
~ has completely different political views
~ if he has kids, unless I am like 30ish and there is a good relationship between the kids, mom, the SO, and myself. OH! and no alimony!!!
~ religion is an iffy situation . . . if the religion is not a cult or non-Christian based, I think I'd have a problem dating them. Being friends with them is one thing, but dating them or marrying them is totally different. BTW, I am not a religious nutcase.
~ some career-choices I would have a problem with. This might seem sacreligious but I wouldn't date a minister/preacher, etc. or a youth minister (dated a youth minister and would NEVER do it again . . . others I know have experienced the same thing I did) (PM me if you want to know more )

I guess to break it down, I'd have a problem with kids, politics, religion, and STDs. I am not a romantic so I would rationalize the relationship before committing further into the relationship.