Too much structure, and people will wait until they have all of the details needed to start contributing, or they will not contribute unless it's "their job". Too much flexibility and they don't know where to start.

I found that it was always better to let documentation and knowledge sharing follow the same principles as code: let it come bottom-up, see what patterns emerge and only worry about structure when the current practices hit a bottleneck. But then again I never worked on any project that I had hundreds of people directly depending on the documentation, so it might not work as a strategy for larger-scale organizations.