I just visited DU, and I second your estimation of their level of discourse, if it can be called discourse. Perhaps 'course' alone would be a better descriptive.
Two thoughts: It's a well-accepted fact that one's ability to write (and think!) cogently, fluently and presuasively is influenced by what one reads, more than any other factor. So what in the world have these lefty whiners been reading???
Secondly, their clunky, Yahoo-like bulletin board messages seem like 19th century technology campared to the FR format, which allows for a running dialog and can be viewed and read much like a newspaper. I love FR, but I probably wouldn't be a poster or frequent visitor if JR used that outmoded system. It's a laugh. I wonder why only FR, of all the blogs I've visited, has this attractive advantage.