Basically, the main thing is that people should have common sense (kind of uncommon, I know).
No nasty language. Have a good attitude. No linking to anything naughty (like illegal downloads or pornographic things). Nothing sexist, homophobic, racist, religiously intollerant, etcetera (use your discretion as to what is and isn't bad- I think people can tell the difference, even if they pretend not to). Don't talk or behave in an offensive or tasteless manner; for example, when talking about say, Motoko, it is fine to mention that she is wearing a skimpy outfit or that you think that she is a beautiful woman, but please don't go on and on about how "hot" she is like some drooling idiot or make other undignified/immature remarks. Likewise, it is okay to have discussions that touch on "mature" themes (the likes of which maybe involved in some anime), but don't be crude and keep it as clean as possible- we do not want to hear something foul described in explicit detail, and we certainly don't want to hear about things like that exciting you. And if you want to talk about "certain" male or female body parts, well obviously there is nothing bad or awful about naming them from a scientific point of view, but it is probably nicer if you just "imply" what you mean in an innocent sort of way. And it's probably nicer in general if not every topic ends up in adult subject matter-world, for that matter.
And also, please remember that people on the Internet are all different ages, will have different backgrounds and cultures, and generally come from all walks of life. Gently help people who seem a bit misguided or young, and have patience with the first year philosophy students; because we all thought we were great when we were new to big ideas. Always be asking yourself, "How would I feel if-" or "How would he/she feel if he/she heard that?" Also, if someone does not behave in an acceptable fashion, unless it is something that's just really awful, be civil about it. It isn't nice to talk down to people, although we all sometimes feel that some people deserve it. Finally, don't be too full of yourself (power-hungry individuals and cocky show-offs are not welcome)!
Besides these few hard-and-fast rules, the rules are really more an "attitude"...
You know what you think of when you think of Jesus? Be like that.
