I have done a number of online courses, and a proper eval of the idea is too dependent upon the individual program/course to be able to create a generalized statement.
in the courses I completed, the discussion board was very lively, with professors, tas, and students actively posting questions, links, and comments.
I would be so bold as to say that in some of these courses, the discussion was significantly better than what is realized in class, and there are a number of reasons for this.
1. forum postings have permanence, so if you say something stupid, everyone will see it for a long time as opposed to forget it as may happen in a large lecture hall. this, at least with my exposure, ensured that postings were well developed thoughts
2. postings can pull in outside media including file attachments and links, so current events and full length documents can be more explicitly referenced, and sourced by the students (not just the professor)
3. students weren't pressured to make comments that would be scored, as happens in many live graduate level courses. I've got a buddy at a top 5 school and another at a top 25 school right now... both of them have to make 1 comment per class session, which is graded from 1 to 5. both have said you have many students who repeat comments or who say things that are irrelevant or poorly developed, just to hit their 1 comment.
the networking is what got hit the hardest, which has been commented on - but, I think everyone involved understood that impact, so we all maintained very open email lines. very much like any other forum, such as this one.
all said and done, I think it really depends upon the people involved. active forum members make many of the detractions seem less relevant or material, while inactive members highlight the weaknesses of online courses.