Comparing Weblogs to Threaded Discussion Tools in Online Educational Contexts by Terry Anderson and Donna Cameron -
This is a well reserached article (by the Chair of Distance Education in Canada and a student of the MDE program at Athabasca University) exploring the difference between discussion forums and blogs.
"Computer conferencing emerged in an era that predates the Web, thus the tendency is for use that is separated from Internet resources and focuses on discussion of readings, or activities that take place off-line or behind passwords... The current generation of blogging tools were not designed for closed class courses and thus the component of teaching presence referenced as facilitating discourse is in some ways more challenging in a blog environment."
The blog is a "freer" environment, and the control of or facilitation of the discourse is a more difficult process. It's this "chaotic" nature of blogs that explains why many academics are so immediately dismissive about it. Those academics that do embrace blogss do so with great enthusiasm; those that do not embrace it are quick to criticize it.