I like some of the ones mentioned. But I can add some from my experiences:

The Disappearing Act. This is the biggest and main one. Kinda like what Seshat mentioned, but I fall victim to all sorts of this one. The no text back, no calls for days, constantly being stood up for dates. Then comes back and expects everything to be ok. UGH

Contacting and always dreaming about the ex - always in constant contact with the ex-girlfriend or ex-boyfriend. You know some people may disagree with this but I for me I couldn't be friends with an ex unless I know for absolutely sure that I am over them. And for one of my exes he constantly told me over and over again how the sex with his ex was the most intense and best he ever had. Thanks douchebag! UGH!

The way he/she treats other people/animals. This I agree with the others have said. How he/she treats waiters in restaurants, cashiers at stores, and animals in general. You can tell what kind of person they are by that.

That's all I can think of for now too. Good thread btw! ::