I shamelessly lift John Wilkins’ comment policy from his Evolving Thoughts, one of my favorite blogs:
This is my living room, so don't piss on the floor. I reserve the right to block users and delete any comments that are uncivil, spam or offensive to all. I have a broad tolerance, but don’t test it, please. Try to remain coherent, polite and put forward positive arguments if engaged in debate. There are plenty of places you can accuse people of being pedophilic communist sexist pigs; don't do it here.