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.)
Well, the content of that particular blog has something to do with that. You need a certain level of sophistication to come up with something coherent to post, and most of the trolls out there know they'd be instantaneously obvious to everyone else on a topic like that.
(I should perhaps disclose that I'm one of the authors of the linked article.)