If you want usable tags, then you first need a good ontology hierarchy. Inventing good ontologies is a problem that's orders of magnitude harder than a problem

If you want usable tags, then you first need a good ontology hierarchy. Inventing good ontologies is a problem that's orders of magnitude harder than a problem of making a filesystem hierarchy searchable. Look at Wikipedia -- despite all this time and all this truly immense planet-size effort, they still haven't figured out a good categorization system!