While watching Coursera's Quantum Mechanics class lectures, which were each about 16 minutes long, I realized two things:
1. I was getting bored with the long length of the videos and after a couple of minutes, tended to stop watching.
2. I wished there were embedded quizzes every couple of minutes. After a new concept was introduced, I found myself wanting a quiz so I could see if I understood it. Even though I could pause the lectures (something I wouldn't have been abe to do had the class been in-person), the fact that it didn't include a quiz every couple minutes made the class feel more like an in-person class rather than how an online class should feel.
In my opinion, it is not enough that you can pause lectures when they are online. You also need to be able to check your understanding with quizzes scattered every once so often.
Another thing that just occurred to me: In my interview with Sebastian Thrun, he said multiple times that at Udacity, he (and his colleagues) don't lecture. When he was at Stanford, he would lecture, but at Udacity it's more problem-centered. When I was watching the Quantum Mechanics lectures, they felt a lot like standard lectures because there were no breaks for me to practice the concepts being taught. The professor was just lecturing, and not taking advantage of the fact that online, you can include quizzes because you suddenly don't have to worry about wasting students time.