We have to make categories useful for commons, and that means turning them into a tagging system (what they technically already are). And that can only be accomplished by getting category intersections to work. That would also mean getting rid of the "most precise category" paradigm.
- Who needs a category "Bridges in Swizerland" when you can tag images
with "Bridge" and "Switzerland".
- Who needs small browsable categories when you can have small browsable
intersection results (and galleries for that matter)
For a tagging system, we also need to change our content providers, the users. I dare say that they can't categorize and they will not tag in a sensible way. That bridge in Switzerland would probably be tagged with "Ginevra", "holiday", "aunt", and "Isabel" (plus a few prepositions). Theoretically tagging is a great system but in the hands of a gazillion amateurs it will most likely lead to missing images, not found ones.
How is categorizing different, you may ask. Categories help us find images that are not categorized well enough and improve them. To be able to look for images which are not tagged well enough is a different matter.