Jon posted in a comment here talking about how one system does more advanced genre work for music.
I really like that idea. I'm thinking of how we could get the benefits of something like that.
Maybe it's not a genre so much as a description of the music. And a piece of music should have multiple descriptions associated with it. Some examples of descriptions could be: contemplative, intense, melancholy. While a piece of music might only have one genre or subgenre, another level of description would be helpful in classifying the music.
Meta-data is going to be one of the more critical pieces of the system, so doing some brain storming on how to extend the "standard" musical meta-data is good.