1- Content and video diversity:

Most of videos are done by a small group of persons.

I suggest allowing people to post their own videos teaching those subjects and add some kind of moderation for the videos. I believe this would increase the quantity and quality of the lessons in this page, not to mention that it decrease the cost of creating new content since its the community itself that creates it. (Just like in Wikipedia).

2- Answer system:

2.1: It turns out that the main difficulty of answering questions properly in khan academy is not actually knowing how to do the thing you are doing but to watch out about what specific format must have the answer. Together with the fact that sometimes you must answer 5 times correctly in a row it can get a bit frustrating.

You should improve that system, detecting when the mistakes are from the format and warning the user to correct his answer format instead of taking the answer as a mistake.

Also you should not lose all the progress if you mess up and type a bad answer once, so i suggest changing it for a "experience" bar that works like this: If you are correct you gain experience and if you fail decrease the experience, if you are on a streak increase the experience gain/loss. 

2.2: Remove the test format or use it less as it can lead to users skipping content they didn't learn.