I read this thread yesterday and was having a hard time coming up with something. There's quite a few things that I don't like in a person, but a lot of the time i'll overlook them if I consider someone a friend, which has now changed because it's gotten me nothing but heartache in the past.

Backstabbing is something I won't put up with.
While most of the time i'm really trusting towards a majority of people, the second that trust is broken it's really hard for me to ever trust them again.

Another thing is fake people. I don't like when people pretend to be something when they're not. Don't pretend to be friendly towards me if you don't really feel that way. I can sense things like that from people. There's only a few, and I guess that's why I'm not really friends with those people.