The knowledge base
It has already been mentioned how the knowledge base of a learning system is better modeled by a graph than a tree.
There is a paper (Aimeur et al, 2000) that describes a way in which capabilities can be mapped in terms of prerequisites and then lessons are modelled as gaining groups of those prerequisites, thereby setting you up for a new set of lessons. It also brings up the notion of different types of links: obligatory, optional, strong and weak contributions. The optimist in me says something like E2's hard/soft links mixed up with the described system might work.
The pessimist in me says that assessment is too necessary to miss out; but also too important and too specialised to provide in such a collaboratively built resource. One option is an assessment builder. Exam papers tend to consist of a set number of questions. Having someone describing these 'meta-questions' somehow a program could produce any length of assessment on demand. The system would naturally be limited in the type of questions that could be asked at first, but systems are being developed all the time.
Not directly linked to this, the Aimeur et al also describe the notion of 'cognitive dissonance' in learning. Cognitive dissonance is a state of mind where one experiences something that does not fit in with the knowledge obtained up till that moment. The brain is then forced to resolve the dissonance by either accepting new concepts and ditching the old or maintaining the old concepts and dismissing the new. The notion being that a learning system could utilise this by sneaking in false information and forcing the student to think on the subject. One advantage of the proposed system is that with the possibility of flawed information in the system the user/student is forced to evaluate the environment.
My own belief is that such a system could be built but finding willing contributors would be the difficulty. Manufacturers of electronic whiteboards have been trying to get teachers to create resources to share between each other in the UK and to my knowledge the uptake has been minimal.