Incorporated subversion by James Farmer takes a look at blogs vs discussion lists in online learning environments (OLEs).
Here's a little about what he says about the limitations of discussion boards:
Discussion Boards "offer little opportunity for users to “project themselves socially and emotionally, as ‘real’ people”...In developing teaching presence in a discussion board environment the teacher has no more capacity than the learner in terms of sustaining discourse or of projecting him or herself as a “real” person."
As a contrast, he notes that "In terms establishing social presence it can be argued that weblogs offer a significant opportunity for users to project themselves as “real” people."
And his conclusion is that:
"while discussion boards may have a role to play in the shaping of future OLEs it is arguable that this role should be complemented by the implementation, within or alongside these systems, of weblogging functionality."