Curt Siffert has a identified a few reasons why blogging as a discussion tool is pretty poor. Over at iWire, steiny piles on but focuses on how good ideas can get lost in the blogosphere.
All good points, and I'll also add that blogs don't centralize discussion. This means that keeping up on all the elements of the discussion is well nigh impossible.
In the physical world, this sort of discussion does occur between cultural magazines and reviews, literary journals, and scientific publications. However, the number of venues is smaller and the rate of discussion is slower. Thus humans can manage, but barely at that.