That's a great idea, and I really like how it looks (and how usable it actually is for a first release).

The only flaw I see is that the most popular questions on SO are those with the most views and upvotes, thus those everyone can understand and appreciate the value of.

This is fine, but it also means that they're the ones most people already know the answer to.

That means I might not use your tool to learn more on subjects I already know a lot about, but it definitely looks like a good way to start learning a new one.