On the freakonomics.com blog, according to Dubner, 40 or 50 comments on a certain post are considered "quite a bit" even though this is a very popular blog with "many thousands of readers" so the number of nature of the comment stream depend on the nature of blog or at least the post.
With an overall number of 117 comments it certainly looks like asking the right question can be one of the best ways to get a nice number of comments. Following are two of my favorite responses to Dubner's trigger question:
on 15 Mar 2007 at 8:52 am # JakeSutton
...I comment regularly on the blogs of people I actually know, but very rarely on the blogs of strangers or on what might be called “public figure blogs”. I think a lot of it actually comes down to basic human shyness. Nobody wants to speak up for fear of sticking their foot in their mouth. ;^)
on 16 Mar 2007 at 7:22 am # yorkd
... Some time ago, Brian C. Warnock posted a question to a Perl discussion group, got no replies and wondered why — the lack of response to his query become known as “Warnock’s Dilema”. Wikipedia has somewhat similiar “stub problem” — incomplete threads, lacking incremental knowledge from commenters. Lower status bloggers are faced with a sortof Fermi’s Blogger Paradox — why have we not been visited? Why hasn’t anyone addressed our question(s)? Is there life out there that understands my queries?