Plug: I'm launching Discoverboard.com for this in a couple weeks
If it grows and that isn't enough to keep it respectful, I will add a $1-2 signup fee for new users.
I'm also going to be spending most of my time recruiting quality people to join the discussions.
But how to promote quality? One thing I would like to see is a forum that cracks down on thoughtless posts, especially hyperbole ad rants. It's not only a waste of time but that kind of behavior spreads like a virus.
Quality is hard. I don't think there's any silver bullet, but you can check out the community guidelines at discoverboard.com/about which hope to curtail the thoughtlessness you're talking about.