I'm not advocating for seriousness at all times. I love humor. Everyone else who knows me in real life knows that humor is a big part of my life.
My problem is that there are times when someone starts a thread, then out of the 10 responses that follow, 9 of them are people with very dry humor trying to make jokes. Instead of witty humor mixed in with substantive conversation (which we all love), it's a bunch of wannabe comedians overrunning threads.
You'll get one guy that makes a mildly funny comment first, then you get four more guys after that who try to make the same joke, but it's even less funny. Pretty soon you've got a thread that's waist high in BS and you don't get anything out of it.
There are countless examples to choose from, but here's an example from one of my threads not too long ago. It was after this post that my activity on CB cut by about half (this was just the straw that broke the camel's back):
http://www.cougarboard.com/board/message.html?id=17403487