My high school had amazing math textbooks. The books had been written by the faculty over the course of many years. Each page consisted of 10-12 word problems and nothing else. There would be no explanation, no section introductions, nothing but problems.

The problems were designed to be just hard enough that you could solve them by making a little bit of a leap from your previous knowledge. After solving a series of leading questions, you would make one last leap, solve a problem, and the book would tell you: "Congratulations, you have just derived the Law of Cosines!"

Because the difficulty was set just right - not too hard, not too easy - doing math homework was like playing a game of Sudoku. ( well, the difficulty was right for me, students who had less aptitude for math tended to hate the books). Also, because you learned by figuring out things as you went, the knowledge stuck the first time. No tedious drills or memorization were ever required.

If I was to design the perfect online education system, it would be like those books. No reading and memorization, just problems designed to be in the sweet spot between boring and frustrating. Online you have the big advantage that you can dynamically adjust the difficult level of the problems. This would actually solve the major problem with the textbooks.

The textbooks are all online here: http://www.exeter.edu/academics/84_9408.aspx

If I was creating an online learning site, I would try and license the material from them. Then I would put the problems online, make the difficulty dynamic, add hints and helpers for tough problems, etc.