The Rules of the Diary Game

What are they?  Is anything fair game?  I assume general common sense guidelines apply.

Common Sense Rules, i.e., don't run a pr0n site in your diary.  Or a really nasty racist diary would seem to be verboten.

But beyond dumb stuff like that, can one talk about whatever they want?  Does it have to be political?

I am curious.  From reading other diaries, it looks like anything goes.