I have three simple rules.
The first: don't post anything illegal
That means the obvious stuff. It also means stuff like links to downloads of copyrighted works (though discussing piracy is fine).
The second: don't post anything my webhost doesn't want me to host
This covers the aforementioned illegal stuff. It also covers legal but undesireable stuff like porn. Basically, keep your images safe for work and don't make profanity-laden posts (though swearing in general is fine).
The third: don't be a dick
Treat everyone with respect. No bigotry of any form. People's right to feel welcome on here trumps your right to spout your ill-informed opinions.
If you break the rules, I'll delete the posts. Repeat offenders will catch a ban.