If there was a mod-filter here, or an upvote/downvote categorizer like there is at Slashdot, things would change. The direct payoff of humour is pleasure, with insight being of a different quality. There's more variety at Slashdot, and people can be more of themselves, for better and worse, and the modding style and layout promotes that diversity. Here, people downmod anything that doesn't add value to the discussion (or strongly opposes one's point-of-view) because you have to look at it, but on Slashdot, participation has it's own inherent value because of the filter.
Furthermore, if people here were identified by their real names, can you imagine how straight-jacketed most would feel, not wanting to be perceived as weird or 'funny' at any point?
Just takin' a stab.
I always upvote something that is funny: maybe here funny isn't smart, which is wrong, or, funny is smart, which can easily be the case, but smart people who aren't funny don't want to acknowledge that others are funny.
Maybe "humour" is upvoted but "funny" is not because it's weird and uncommunal. So humour is often not appreciated because it gets too easily percieved as funny amongst the ultra-seriousness.
Maybe being in serious concentration, which gains the upvotes, people forget about their funny side in both reading and writing.