Very subjective, I'm sure it's not 100% true in every case, etcetera. But I think these are good guidelines.
1) Anyone who out-and-out says that they're not what you're looking for. Someone who says that he's not in it for a long-term relationship or she's not good at being faithful... probably means it. I'm amazed how many people ignore this.
2) Anyone who's "the jealous type." If they're jealous before you're even exclusive, they're going to be bugnuts obsessive down the road.
3) Anyone who has a major sexual incompatibility with you. Minor ones can be worked through, vanilla and willing to experiment can definitely find love with kinky and patient, but if one partner has a fetish that they must act on and the other one will not try it, then it's not going to work. I don't think it's fair to assign blame in this kind of case, to call either partner a demanding perv or an uptight prude, it's just that they're not right for each other.
4) Anyone who states they think most men/women are assholes/bitches, but you're different.
5) Anyone who won't use a condom. Yeah, I know they're hell on boners, but they're also extremely non-optional until you have a lot of trust in the relationship, not to mention STD tests and birth control. And any woman who refuses condoms has something funny going on.
6) Anyone who makes you feel like you're their mom/dad. This is a trap I used to fall into a lot--the "fixer upper" boyfriend who just needs a little encouragement to be sociable or take care of himself or put in job applications. This kind of person never gets fixed; they become simultaneously dependent and resentful as you run onto your last nerve.
7) Anyone who's hard to get ahold of. I know quite a few people who are tons of fun when they're around, but they don't check their email and they don't charge their phone and it's freakin' impossible to find them when they don't feel like being found. These people do it on purpose and they don't get better just because you're dating them.
8) Anyone who has a problem they're not taking care of. Someone with financial/health/mental health/family problems isn't necessarily a basket case--someone who isn't paying their bills and taking their meds is. Baggage happens to the best of us, but the best of us deal with our baggage.
9) Anyone who has a tale of woe. I don't care that everything in the tale was totally unfair and random and not their fault--the longer the tale of woe and the earlier they tell it to you, the more full of shit they are.
10) Anyone that no one understands but you. Especially if they actually say that.
I've dated about nine of these. It never ended well, but more importantly, it never middled well. I can look back and see a long period before the breakup where I wasn't enjoying him so much as dealing with him.
I think #10 is the biggest trap, especially, but not only, for very young women--there's a sort of romance in thinking that the common folk just don't see his/her inner beauty. When I volunteered at the Humane Society, I noticed that nothing adopted faster than a three-legged dog. A beautiful, well-trained, affectionate young purebred would sit in a cage for weeks while people (especially, but not only, very young women) went all wibbly for a grouchy unhousebroken mutt on three legs.
Don't date a three-legged dog.