1) No transphobia/homophobia/racism/etc. This always becomes a point of contention because some people can't understand why we can make fun of some people, and not other people. Essentially, if you think you're being funny by making fun of someone who is trans, that sort of makes you an rear end in a top hat. If you're making fun of someone for eating an entire block of cheese at a train station, that is funny. One person is subject to institutional and social vulnerability; the other person is eating cheese in public. If you can't tell the difference between those things, then I'm sorry. I'm going to be incredibly loving strict about this, including colourful language. If I cannot tell if you're joking or not, I will assume you're not.
ADDENDUM: This rule also encompasses a prior rule, which is that people who are a little fat or weird-looking but who can be otherwise categorized as unremarkable do not belong in this thread. A good rule of thumb is that if you saw this person at the grocery store, would you immediately drop all your groceries and call a priest or a rabbi? That person belongs here. If instead your reaction would be, "Ha-ha, nasty," but continued picking out a pasta sauce, probably not. Do not be goony about this rule, please.