I have always used categories to force structure on people making posts. I would create X number of categories and all content must be filed under one or more of those categories. As such, I could have a simple menu that is controlled by me to dictate the different categories that you can browse. Then, as a way of offering the contributors a way to determine their own organization, tags can be input by the contributor as they see fit. I do not dictate what tags can and cannot be present. By building a tag cloud or tag search, the content can thus also be structured by tags.
In sum, I use categories to force structure on contributors and tags to allow them to dictate structure.