I don't mean humor in general – which is great if you can include it – but the one-liners, puns, and feigning ignorance jokes.
I don't know. I guess I'm worried about that scenario that plays out all the time where someone puts effort into making a good argument hoping for deserved admiration, but then someone else can come along and crack a simplistic joke and in one second get promoted right past the first guy.
If that scenario plays out it not only discourages valuable discourse, but it encourages the opposite. Jokes are easier than thoughtful comments. So if they're rewarded just as well or better, than people will choose that route.
And a thread full of attempted jokes gets noisy fast.
A high signal/noise ratio is very important for some. I'd bet it's very important for most here. Tokens of information (a post, an article, etc.) often make it to the top of the heap in many user-driven environments simply because they are (1) super-easy to digest and (2) appeal to most. I left digg because that content won out, and then the same with reddit. I don't want to see a funny picture of cats when I come here (I have rss feeds for that), I want articles that are difficult, and comments I have to chew on.
'Karma' does work, though it is just a number.