Here's a moderator opinion (not the moderator opinion, of course).
I, personally, think that there is considerable truth in the "aggression theory of humor". The best kind of joke is transgressive -- something that dances on the very edge of acceptability, brings its audience to the brink of sadness and hatred and disgust, and pulls back. That's the kind of humor that really reveals something to the audience. It's also very hard to get right: go just a step too far and comedy does just turn to pure sadness and hatred and disgust, and you squander all the trust you've built up.
When interacting with strangers (e.g. on a large-ish Internet forum), most people are absolutely incapable of getting it right. They don't can't read where the lines are so they either pull the punch so hard that there's no humor there anymore or go sailing all over the place and creating negative reactions left and right.
Most people also joke selfishly. When they get a negative reaction, they start equivocating about how the onus is upon the audience to "take a joke" -- rather than the onus being on you to know how to make one in the first place. And then you get snippy arguments about who is entitled to tell other people how to feel.
So, when I see someone try to make a transgressive joke and fail miserably, Official Bad Stuff happens. It's a way of heading off that big ol' Internet argument about who gets to tell other people how to feel.
And, of course, sometimes folks fail so hard I'm not even sure whether it was supposed to be a joke anymore. We can't read your mind.
It's your responsibility to think before you post. Nobody else can do that for you. That applies to attempts at humor, too.
-- Alex