There is an academic field around this called intelligent tutoring systems (http://en.wikipedia.org/wiki/Intelligent_tutoring_system). The biggest company with an ITS, as far as I know, is Carnegie Learning, which provides entire K-12 curricula for it: books, teacher training, software. CL has had mixed evaluations in the past, but I think a fair conclusion at this point is that ITS significantly improves learning outcomes when implemented in an environment where they are able to use software as it’s intended to be used (follow the training, spend enough time, etc).
As far as I know there isn’t anything quite like this in a widely deployed online system with community discussion as you suggest. Grockit (http://grockit.com) is a social test prep site that is familiar with the ITS community and uses some principles. Khan Academy is continuing to improve, but I can’t say whether they will reach the state of the art as far as intelligent tutors go. I’d say there’s definitely an opportunity for more ITS in online learning now, but it isn’t easy to build.
The Wikipedia article is OK. One example of a recent paper is http://users.wpi.edu/~zpardos/papers/zpardos-its-final22.pdf which also shows some of the human work that goes into modeling the knowledge domain for an ITS.