The first one that comes to mind is if she mentions feminism too early on or too often. I don't mind feminists so long as they aren't retarded but if they mention it early on it implies they're one of the radical types and if they talk about it a lot I know I'm gonna get annoyed. Another is if they admit to having feelings for an ex (OkCupid handily has a question about this which many girls seem to answer truthfully). Another is if they've cheated more than once in the past, or if they cheat with me (I'd still be in an open relationship or FWB in those cases). Oh yeah, STDs and not using a condom with other men is another one. I don't care if she sleeps with other guys before we agree to be exclusive, because I'm gonna sleep with other girls up until then, but I get preferential treatment, so regardless of what birth control she's on, she's not allowed to give head to or have unprotected sex with other dudes ('cause I don't wanna be thinking "can I taste cum?" while kissing her or going down -- if I found out a girl was having unprotected sex with or giving blowjobs to other guys and then kissing/having sex with me, heads would roll).
I'm not really bothered about specifics. I don't get how people can have red flags like "excessively overweight" because to me, a red flag/dealbreaker is when you're attracted to someone but they have a flaw that's unacceptable. If someone is boring, obese or mistreats me, then I'm not attracted to them in the first place.