I really want to respond at length to Tim Bray's post What Do Tags Mean?, but I've got work to do. I'll limit myself to two points - Tim says (regularly) "There is no cheap metadata." I don't think that's true, or particularly relevant. For example, all my outgoing mail contains my name - data or metadata? What's the cost? In general this question should be balanced with: what's the benefit?
If you have high benefit/cost, is that cheap or merely good value?
The other point is regarding tags, taxonomies, namespaces, structured vocabularies and personal ontologies. Tim's right about the difference between Technorati's flat approach and Atom's potentially structured approach, but there's been a lot of work done in this area.
In a summary form, it's all there in Morten's FOAF Output Plugin.
Oh, and a different angle is Pietro's Delicious Mind Map Maker, which shows how you can elicit structure from the flat stuff.