Moody, inconsistent people. I like to know where I stand with people. I hate it when someone treats you differently from day to day. Like they are your best friend one day and then the next day they pass you by without saying hello, or they get upset with you for some odd reason but won't discuss it with you. Or people whose moods just seem to change instantly. I won't even be friends with people like that.
I also steer clear of people who are overly negative, depressed, or constantly wallowing in self-pity. Obviously everyone feels like this from time to time, or even for extended periods when facing difficult situations, but I'm referring to people who are like this the majority of the time, seemingly without good cause.
Other ones would include alcoholics (I don't mind drinking, but not every day and to excess), criminal activity / parole violators, couchsurfers, mooches, the happily unemployed, people with too much personal drama / baggage, and disloyal people or cheaters.
Also, self-absorbed and inconsiderate people who leave you hanging when you are supposed to meet up and don't even call you to say they will be late or even have a good reason for it when they do finally show up. The first time that happens, I'm out the door. It's a sign of a general lack of respect and common courtesy for other people.