I've recently spent a lot of time looking at tags. I'm fascinated by how creative listeners are when it comes to inventing new tags for the music they care about. And I'm fascinated by how small communities adapt these newly invented terms as quickly as they tune into a new radio station. This constantly changing and seemingly endless vocabulary of words to describe music makes me wonder if research on automatically classifying or tagging music will ever be able to catch up.
If you think research is catching up, check out the 22,703 shades of metal, or the 11,929 shades of pop, or the 29,940 shades of rock at Last.fm.
See also Paul's experiments on creating a taxonomy of metal and gothic tags.
See also Mike's recent results on autotagging of music (which includes some impressive numbers).