|Be fucking polite.|
Calling someone an idiot is fine. Lots of people are idiots. Just use caution when moving much beyond this level of invective.
A few specific things that will get your comment banned as soon as I see it: the words "cunt" or "faggot," references to sex organs, gratuitously nasty personal attacks.
I'm not going to go back and delete old comments, but rather leave them up as a reminder of why we can't have nice things on the internet. But no more of this shit.