I think you've missed another very important part out. There's a very large, and extremely active community based around each course. Some are necessarily more

I think you've missed another very important part out. There's a very large, and extremely active community based around each course. Some are necessarily more active than others. Most of them have the actual teachers participating and I've witnessed the forum evolves into a peer-driven help system. Peter Norvig's activity on his course's forum was astounding and I've never seen a more helpful and active online community before. I haven't seen that with other online courses. OCW had nothing in terms of community engagement. Coursera does have a forum for each course, but they are very dry and the engagement factor feels missing. I haven't had a chance to take an offering from MITx yet. I have seen a few interviews where Udacity staff or teachers have said they deliberately wanted to engage the students and to encourage them to become active and support each other and I've seen that it's quite a big boost to the rest of the offering.