a sense of humour
friendships with laughter are the most cherished ones

open minded
it's not necessary to be pro-everything, but being open and respectful of others lets people be themselves

accountability
if you say you're going to do something, do it. If not, then at least be responsible enough to let people know when plans change---it's rude when you leave people hanging.

be nice
seriously, no one wants to be friends with someone who's a jerk. 24/7. =P