This seems like an amazing concept and I have been thinking of it ever since I saw KhanAcademy knowledge tree.

I can see this being combined with Arbital's Lens (Same thing explained in multiple ways - from a simple explanation for a 10-year-old to a rigorous explanation for a mathematician)

I always considered this the best way to actually progress in my learning and I still see it as the next big thing after Wikipedia, once it's done correctly.