From what I've seen, a bit of reasonably mature humor seems acceptable, it's just multiple posts devoted to jokes derailing the conversation where it's frowned upon. I quite like this system, as it prevents Reddit-esque threads with the same tired memes being repeated back and forth ad nauseum, but still allows for a bit of levity to lighten the conversation and provide some mental refreshment.

And yes, I realize I just called a Monty Python and the Holy Grail reference mature.