My earlier comment about traffic really boils down to the fact that it is hard to police a community of blog commentators. However, if you look across the “pond” to the web-based forum, they often don’t have the same kind of problems. Forums like phpbb offer much tighter and fine-grained control over the userbase.
However, there is a lot of overlap between a forum and a blog. Topics are categories; posts are posts, replies are comments. You can essentially map the two onto each other.
I had posted on this some years ago, but others have also been thinking about it. For example, there’s a plugin for phpbb called phpbblog.
UPDATE: There has been substantial interest in creating a plugin for WordPress that uses the WP database for the forum user db. Also, the forum software in use at wordpress.com (bbpress) itself actually boasts complete user integration. But this kind of misses the point – I am talking about a blog and a forum as different “skins” to the same discussion content, not two separate content spheres intersecting solely at the level of the userdb.
 my previous posts on this topic below the fold: