Why are people on message boards so NASTY?

Heaps of reasons which have been mentioned here already. Another huge factor is that the written word is easily misinterpreted at times due to no tone of voice or facial expressions (unless you use emoticons etc). I have seen people who agree with each other argue tooth and nail just because they are saying the same thing in a different way.

Also doesn't help when you have people calling someone names just because they disagree with what has been written.

I've also seen Mods and Admins create turmoil by being too extreme with rules and regulations. Sometimes chats go off the original topic a little and I have been on sites where that is a big no-no, making for a very sterile environment and can spark arguments just as easily as one like the original IMDb where people could say and do anything they wanted to.