For an example of blogs with very high quality comments, see, e.g. http://www.nature.com/nature/journal/v461/n7266/full/461879a.... It's an essay describing the use of blogs and blog comments to solve a difficult open mathematical problem. The average quality of the comments made in that project - more than 800 comments in all - was extraordinarily high. Several strongly research-oriented blogs in mathematics and physics routinely have very high quality comments, e.g., the blogs of Richard Lipton, Terence Tao, Tim Gowers, and the n-category cafe.
(I should perhaps disclose that I'm one of the authors of the linked article.)