As Zaarock said, it's not a completely cut-and-dry rule.
The basic premise is that instead of posting on the thread, yelling at people for breaking the rules, etc. etc., and expecting that a moderator will come along shortly to take care of it, we'd prefer that you use the handy "Report" button, which is right below the Player Statistics button.
This creates a notice for moderators that gives a link to the reported thread along with the reason that you're reporting it, making it a lot easier to find the problem in the first place.