specially if it includes an insult about an acceptable target.
Sounds like you're referencing Hanson, who references benign violation theory. Who is the target of insult in the Garfield quote? The characters, the author, or his readership? Which would be acceptable? I suppose the case to make is that he's insulting his readership and the quote is intended for non-readers.
The Computer Theory of Humor looks interesting. The details in that entry are a bit unclear, but it appears to characterize our humorous response as being a bit of a kludgey and excessive hack to remove our focus from the incorrect interpretations we were forming prior to the punchline about pink elephants.
I think I came up with "acceptable target" on my own--at least it felt like I was doing original work while contemplating the highest voted quotes.
My general theory of humor is that it requires benign surprise-- there are many sorts of humor that don't involve insults.
Both benignity and surprise are highly contextual.
Davis was talking about actual hitting in his comic-- it's about a cat with a considerable will to power.
Also, Hanson's theory needs to be elaborated to include self-deprecating humor, though maybe counter-signaling is enough to do the job.
Reading the recent list of rationality quotes arranged by karma underlines the popularity of funniness, and being funny should probably be included in the pursuit of awesomeness.
My best guesses about characteristics of humor: If there's a word which makes the line funny, put it at the end. Phyllis Diller recommends that the word should end with a hard consonant (t or k).
If you can make a surprising statement extremely concise, there's a reasonable chance it will be funny especially if it includes an insult about an acceptable target.
Quasi-quote from Jim Davis, author of Garfield: "If I can't think of anything funny, I have one of the characters hit another." Any other principles of humor and/or methods for cultivating the ability to be funny?
ETA: The most recent thing that struck me as very funny-- how does it fit into the theories?