It isn't about having, or not having, a sense of humor. It's just that Hacker News culture prides itself on maintaining a certain decorum to the discussion here... and it doesn't include internet memes, Chuck Norris jokes, or a lot of the other frivolous stuff that's accepted on Reddit, Digg, Slashdot, etc.

It's not that people here are stuffy, or humorless, or anything; we just want HN to be different, and not decay into a festering cesspool of malfeasance like Slashdot or whatever. There are plenty of places one can go to get that kind of stuff on the 'net. Here at HN, it will usually be downvoted mercilessly.

You're right - posts with nothing but a joke aren't really acceptable here. We want to discourage that sort of thing.