I've been a member of a few forums over the last 10 years or so, and have never read any of the rules, but I figure that if you want to get on with other members it's just the same as real life, you should always respect another persons opinion, don't insult, curse or make personal remarks about other members and always try to be polite
Unless of course there is a troll on the loose, then none of the above applies and you can let rip
It's always worth remembering that most content is open for the worldwide public to read, that includes women, under 18's and old people, and religious people. I wouldn't want my kids reading curse words they might have stumbled across online, so I'm not going to swear or curse online so that someone else's child might find what I've typed.