coercion :) LibraryThing is the only application I know that allows this with tags. Flickr and delicious don't do anything. e.g. hierarchical organisation/tag linking - AFAIK no tag-based thing does anything like this.
Ok, let me make a provocative statement:
Commons image categories are useless the way the are applied today. The hierarchical organisation does not work. Full stop.
It is currently almost impossible to harvest information from the category tree. Database requests to find common super-categories and perform boolean operations are not feasible, since they run on the order of minutes.
Life would be easy for application developers if categories were used as simple tags. We'd get faster db queries (no more category crawling), making real category intersection possible (or let's say plotting of image subsets on GoogleMaps, i.e.: all pictures except food items).
Life would be simpler for end users. Boolean category operations would provide a natural and powerful way of finding content (think AND, OR, NOT).
Life would be easy for uploaders. No worries if you found the right subcategory. No accidental 'overcategorization' because some category happens to be up the tree of another category.
For hierarchical organisation and browsing we could still use galleries. Just stop the subcategory sprawl and use multiple tags.
difference is, with everything else it's *easier*, and often that seems to be enough.
Yes. If it is easier on the user, easier on the application developer, and easier on the uploader, than it is more than just "enough".