Before I had kids I looked for friends with the same soci-political beliefs as me, friends that had something in common with my own background (neighbourhood, school, family circumstances), friends that had similar taste in books and movies and friends that loved to eat at restaurants as much as I did.
After the girl was born I looked for friends with similar parenting philosophies, friends that also co-slept, cloth diapered and breastfed and friends with babies who never slept and would understand why I was so tired and strung-out.
Since the boy was born I have refined my friendship criteria. Now I only look for three things in a friend. Can I still tolerate to be around them after a night of disrupted sleep. Do(es) their child(ren) drive me crazy after being in contact with them for an extended period of time. Will they listen to me complain about the girl, the boy, the husband, myself and still talk to me afterwards.
That is the making of a good friend. The best kind.