There's no hard and fast rule with this, and we can't really give examples because they're always removed.
But basically, any discussion about: illegal stuff, things in bad taste, immoral activities, abuse, stupidity, etc...
Think about this: You're at your girlfriends place for dinner, if there's any topics you'd be worried about discussing in front of them, they probably won't be welcome here. That said, it's up to the disgression of the individual mods/admins to determine just what is acceptable.
As soon as we lay down hard rules, you get fools that go out of their way to flaunt them, so that's why we work the way we do... some things are allowed to slide, others are killed before they get out of hand.