- year-performer-composer-star rating
- genre-year-song title
- album title-song title
- ...
You get the same problem almost everywhere. Is it year-project or project-year or maybe customer-year-quarter-project?
Yes, for many tasks, a tree is a good way to structure information. The problem is that the choice of tree can be widely different for different tasks (for example, 'that song you played last week', 'that minute-long guitar riff by X')
I think a hierarchy should be like an index in a database: set by an expert to match the most common access pattern, but not enforcing other access patterns. Hierarchical file systems are good at the former, but atrocious at the latter.
And we sort-of already have WinFS, in the form of full-text indexes on file systems.