It's not that we're humorless. It's just that we've witnessed what happens in communities where you jokes are encouraged in lieu of actual discussion.
Pull up a thread on programming.reddit.com or slashdot today and you'll find a 50 comment tangent devoted to silly puns. Often there will be four of those tangents to scroll past before you can find discussion about the actual article.
The quick and effective solution to this is to simply disallow silly, jokey comments. As you'll note by reading a typical thread here, it works quite well.
You can just hide the thread (*in Reddit). For me that's ideal, since it keeps these people busy when they could be adding noise. If they had something worthy to contribute, they would in another thread.
ITOH, people participate in both silly puns and important discussions. It's not good practice to downvote people who simply post stuff you're not in the mood to read, or not interested.
Humor tends to crowd out actual discussion. Why bother typing out a 5 paragraph comment on your thoughts on the article when you can just contribute to that pun thread or just post a quick one liner? In a forum where jokes are encouraged, they tend to take over the actual discussion. Pretty much anyone can contribute to a joke thread (especially when the bar is as low as just repeating memes), but not everyone can hold an actual discussion. Thus, you wind up with a lot more joking about a topic, and very little actual discussion. It vastly decreases the signal to noise ratio. While humor is easy to find on the internet, actual discussion is much harder. I'd like to keep HN focused on discussions.
I've never seen anyone downvoted for an otherwise substantive comment that included humor. However, if a short, pithy comment offers nothing besides humor:
1. It doesn't gratify one's intellectual curiousity.
2. It's probably not original and probably not even very funny.
