As far as I remember, the rules on offensiveness aren't exactly too dissimilar to what we see in UK law ie: you can't make comments that offend someone based on:
their gender, religious belief, sexual orientation, ethnicity, race etc.
Also, we don't allow you to insult other members on the forums
To take "religious belief" , as far as I understand it, it's absolutely fine to say "I hate x religion because I believe it promotes y" but to say I hate all x, because their religion tells them to do y, is a different matter entirely
Basically, the only real extra rule we have on offensiveness, that UK law doesn't really already cover, is that you can't go and insult people. To allow people to be racist/sexist etc... we'd kinda be breaking UK laws
(And it's a UK based site)