Just off the top of my head, not to be so judgemental.
I'll happily discuss religion with anybody, regardless of their beliefs. But 99 times out of 100, an athiest/agnostic will quickly resort to insults and try to offend to try make their point. The man in the sky, fairytales, etc... Basically all it accomplishes is it shows how immature they really are. Jesus may love them, but I think they're douchebags. So that in and of itself makes me a poor Christian.
On these football boards, I've run across ONE guy who would put themselves in the athiest/agnostic category that can discuss religion without resorting to childish tactics. E_D Guapo over at the DCC. I've learned a lot from his perspective, and actually enjoy discussing it with him.