Wow, there's a LOT of those for me.
Political beliefs and religion are huge. I'm not very religious and I can't stomach someone who is, or anyone who has political ideals I disagree with, they're not even dating, let alone marriage material.
Race is a factor because while I don't want to see anyone get treated badly for the color of their skin, I like my own kind and want my kids to look somewhat like me.
Finally, a sense of humor similar to mine is a must, as is a level of confidence that will keep him from turning into a mealy-mouthed, jealous, stalking basketcase. He also has to be at least somewhat intelligent because I can't deal with an idiot living in my house.