Dan Meyer said:

Teachers are a great medium for lots of things that a YouTube video isn’t. “Conversation, dialogue, reasoning, and open questions,” as I put it in my post. If you, as a teacher, aren’t taking advantage of your medium, if you’re functionally equivalent to a YouTube video, you should be replaced by a YouTube video.

via http://blog.mrmeyer.com/2012/what-silicon-valley-gets-wrong-about-math-education-again-and-again