I just want to make it clear that this should not be an attempt to "see the future" by creating all the possible categories there might be need for. We should only try to organise the content that exists, because it is not possible to predict what our needs will be down the road. We should be primarily concerned with putting the content we have now in its right place. After that, if we know something will be needed and there is a risk that it might get misplaced if let alone, we can consider other categories. (This does not include categories that need to be created in order to put content in its right place)
Also please understand that none of this is ever concrete. If a category doesn't work out, it can be changed or replaced just like anything else in WikiEducator. Members should be expected to create their own categories as needed and preferably along the guidelines created by us and the style guide workgroup.
After my recent foray into ontology, prompted by the Shirky article  (related discussion thread, I understand how NOT pre-determining the category structure, and allowing users and authors to create the categories and connections that they need, creates a more powerful and effective structure for finding what you want.
Nevertheless, we should try hard to word whatever top categories we come up with to reflect our best sense of what will be most helpful to the community, although I suspect it will take some trial and error ;).