I have now ventured a bit into the Massive Open Online Course (MOOC) world on Duolingo, Udacity, Coursera, and Code Academy and have a few opinions. Spanish of Duolingo was great. CS 101 on Udacity, which I'm taking mostly to learn Python, is also really excellent. It's very early, but I'm a bit less impressed with Caltech's Galaxies and Cosmology on Coursera.
Interactivity is the key to success, I think. Watching a lecturer is about as uninspiring on video as in a classroom. I call it close because the modest suspense of waiting for the live action prof to trip over his eraser is balanced by one's ability to stop the video to get another beer.
Hemingway, or somebody, said that if you bring a gun onstage in the first act, you need to shoot somebody with it in the second. I feel the same way about equations in physics classes. You can do that with language - translate this - or computer stuff - code that - and you need an interface to do the same with equations. Caltech's G&C course desperately needs a live interface explore the equations introduced, and some excellent graphics. The technology is there with Mathematica or Maple, but somebody needs to bite the bullet to do the dirty work of developing a real course.