For an example of blogs with very high quality comments, see, e.g. <a href="http://www.nature.com/nature/journal/v461/n7266/full/461879a.html" rel="nofollow">http://www.nature.com/nature/journal/v461/n7266/full/461879a...</a>.  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.<p>(I should perhaps disclose that I'm one of the authors of the linked article.)