If the goal of discussion is to increase learner's critical thinking skills then online has some major advantaged over the traditional classroom. That said, there are still significant skills to leading a discussion whether it is online of face to face and not a lot of places to get training in those skills.
Watched a keynote of Tony Bates today - we've spent a lot of time thinking about online course design, what to put online and how to present it. We need now to think about the classroom. What goes there, why, etc. Also what to put online should be thought about at the program level, not only at the course level.