The problem of music metadata is the relations between things are too complicated. It's not a simple tree. It's a very convoluted graph. If you make the schema

The problem of music metadata is the relations between things are too complicated. It's not a simple tree. It's a very convoluted graph. If you make the schema right, it's too complicated and people will not use it, because they just don't care that much. If you simplify the schema, it's more likely people will use it. However, if it's too simple (artist/album/title/year), you end up with many many inconsistencies and duplication. Finding something in between is nearly impossible. With MusicBrainz, we've tried to design a strict schema that works for most things, but then you need to find people to actually enter data in that schema. Wikipedia is on the other end of the spectrum, everything is free-form and some structure is slowly emerging from that, but it's far from universal. Structured metadata is just hard for people to manage. Unless they are geeks and they really really care.