The first great lecture I can remember watching on Youtube was a Harvard ethics class by Michael Sandel. I thought it was fantastic; way better than sitting in my community college classroom. I suppose there would still be a need for Q&A and shared student interaction since often we don't know the smart questions to ask and other students do. Still, this could be provided at a fraction of the cost online. An expert would still need to host the online classroom, but having not wasted their time giving the lecture, they can spend that time engaging the students. Best would probably be having students teach each other through discussion with an expert merely keeping things on track.