Signal to noise ratio.

Everyone has their own threshold as to what's on topic for the sub-forum (Road, Track, A&S, ....)

Also, a lot of folks get really comfortable in a sub-forum (Road Cycling for example) and want to post everything there forgoing other, more appropriate, sub-forums. "How do I remove my Bottom Bracket?" is not a Road Cycling specific problem. So, stuff like that would best be directed to Mechanics. The same goes for off-topic threads.

I know that the humorous stuff is a tricky one. If you guys are comfortable here and know the members and the mood, it's easy to tell a joke or make a parody thread. Doing it in another sub-forum like Jokes and Humor or FOO might be like walking into a room full of strangers then telling a joke. I guess there's a fine line somewhere.