First, let me say I am relatively new to K5. I don't remember when, exactly, I started an account, but I know it was after the site was down. I, too, still read /., but I don't post there nearly as often as I used to. Usually, the only comments I post there are ones that I think are funny.
I have sat, typing a comment, numerous times as a funny response ran through my head. But, after seeing the response on K5 to the infrequent humor (or attempts thereof), I have refrained. To me, it is unfortunate. A funny comment can help all of us have a better day. Sure, the primary reason people come here is not humor, but does that mean people should discourage it? Not in my opinion. I, for one, will gladly rate up funny comments just as I would rate up intelligent ones.
Another thing on K5, that reshippie pointed out is the length of comments here. People tend to write relatively long, involved comments here, especially when directly responding to a story. They tend to get shorter when you start getting into responses to the comments. From my short experience here, it seems to me that longer comments tend to get a more generous rating than shorter ones, and humorous comments are often short. A one-liner is, by definition, one line. It doesn't always belong in with a couple of paragraphs of commentary or editorial. People can rate comments with whatever criteria they want, including being more generous to longer comments. But my view is that a comment does not need to be long to be valid (as I go on and on and on).
I think we need to appreciate whatever humor we can get. Some might argue, that is not what K5 is for. I think we can all be sure that, no matter what happens, K5 would never turn into a humor site, and since nobody can post anonymously, I doubt it will ever get overrun by trolls. Also, looking right above my comment composition box, I see the words that ensure K5 will not become like slashdot:
Spamming is not tolerated here. Any comment may be deleted by a site admin, and all spammers will be deleted. This is fair warning.
I think it is safe to say, yes, we like humor. If you don't like humor (or it's not funny), continue to rate it down. But just because you're on K5 doesn't mean you have to be serious all the time. Humor is good, especially when it is spontaneous and unexpected.