Having a (very) large digital music library, and being a regular listener, classifying my tracks and albums for my needs has always been an epic quest without any perfect solution. The best thing I've found so far is foobar2000's media library and its various treeview visualization plugins, that allow complex custom sorting, filtering and display (think of it as scriptable iTunes custom playlists).

Back to the point... I recently found two related articles that develop many interesting observations and concepts around the theme of classification of digital items :
Ontology is Overrated: Categories, Links, and Tags
Hierarchy vs Facets vs Tags

Basically, they are explaining how "real world sorting" (folders and shelves) appears as powerless when you try to use it on a large and heterogeneous collection of items. Tags and facets appear as more flexible and powerful solutions (see Nobel prize winners as an example of facet-powered browsing).
That said, new problems come with facets and tags : they fail to give hierarchical information to the viewer (we all tend to organize stuff hierarchically when we're lost... how about living with a classification system with no hierarchy at all ?).
What's more, the act of tagging items independently deprives us of the mental effort we do when we're grouping and sorting stuff to put them into folders. As a result, we have a much poorer mental view of our tagged collection than we would of a classified collection, but we can access its contents much more efficiently thanks to the engine.

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This post has been completed while listening to : nothing !