I tend to appreciate wordplay. I enjoy humor referring to something else, when I know the reference. I like humor that requires being well-educated and/or well-read. I like it to be complex. I like it to build to the payoff.
Then again, sometimes it's all about the poo, or the double entendre. I can't even pretend that I'm so sophisticated and classy that I don't end up in the gutter sometimes. In fact, sometimes it's scatological wordplay that you have to be well-educated to understand. I think the funniest thing I've ever heard was "Peristalsis, my ass!" I said that to myself years ago, and I'm still laughing about it. I can't possibly explain it - either you get it, or you don't. Mostly, people don't.
That's another thing about humor. It's very situational. It's not just about the timing, which of course matters. There are a great number of things that fall into the category of "you just had to be there." Some really aren't that funny, but the feel of the situation made them so. Some really are hilarious, but explaining the context takes so long that it's not worth the payoff. And sometimes, the story can be made funny, or even funnier than it was in the original, but it's all about the delivery. That's generally my aim in storytelling. I think the best I've ever done is probably the body glitter post.
I've spent a fair amount of time listening to people talk about humor. In January, I went to a panel on Humor in Fandom, made up of humorous fans. The other weekend, I went to a panel on Humor in SF&F, with authors talking mostly about how they write humor. But speculative fiction also has the chance to talk about humor, since it often deals with non-humans of one sort or another, who either find different things funny, or try to learn about human humor. Consider Heinlein's The Moon Is a Harsh Mistress, wherein Mike the computer develops a sense of humor, which matures over the course of the story. Look at Robert L. Forward's aliens. I don't recall whether it was the cheela of Dragon's Egg or the flouwen of Flight of the Dragonfly/Rocheworld who told bits of mathematics as jokes (probably the flouwen, then). Even Piers Anthony touched on this, in the Phaze books, when aliens competed in the Game. In fact, I think you could make a good case for how alien a species really is being dependent on how similar or different their humor is, once translation is accounted for.
And isn't that a little bit of how we evaluate the people around us, too? Our friends mostly are the people who laugh at the same jokes we do.