Have you ever read a math textbook and spent half an hour trying to understand one paragraph? Usually everyone else is stuck on the same part. Leverage the crowd's knowledge with contextual comments.
What are contextual comments? You can see an example of them in The Django Book. To be clear, this is completely unrelated to Django; I just wanted to reproduce/steal the awesome commenting system that djangobook.com uses.
The meat of the contextual commenting system is in commentable.js. Unfortunately, it is not completely separated from this GAE app. One of these days I plan to do that, but if you'd like to take a shot at it go ahead. I would love to see the results.