Gawker finds making it harder for comments to be seen leads to better comments
niemanlab.org
niemanlab.org
The best comment model I've seen is the NYTimes, which combines moderation and featured comments (the user recommended comments are too heavily biased towards what was posted first to be useful).
http://www.evanmiller.org/how-not-to-sort-by-average-rating....
In fact, the proposition is that rewarding good commenters with higher visibility will increase quality. Makes sense, and it's nice to see some quasi-empirical evidence for it.
That said, it would be interesting to have a graph that shows the engagement rate for users; something like Comments for every 1 Million page views.