Red flags for me are when the woman says she's "picky" (means she'll never be satisfied with one man, she wants attention from loads of dudes), or if she says she's "traditional" (means she expects you to pay every time and ain't gonna be putting out any time soon.)
If you see that she's online like every night, or if she's been online for ages.. means she likes attention from lots of men, usually is quite a prude or control freak if you actually end up getting to know her.
These are generalizations, but ones which are accurate enough in my experience that I no longer bother with them if I see them.
If she says "friends first" anywhere in the profile, I immediately close the tab.
I also don't date religious chicks or chicks whose first language is not English, but that's my personal preference.