Back when I used to listen to music on pieces of etched plastic, I had a simple classification system. Music was slotted into one of "Chilled", "Techno", "Hip hop", "Rock/Pop", "Classical", "Jazz", "Other". There could easily be overlap there, but because I was doing the classifying, it made sense to me.
When I started listening to mp3s, I carried over this basic system, with Rock and Pop split to separate categories and a couple of new ones added: "Reggae", "Radio" (for podcasts), "Country" (for Johnny Cash). My musical tastes got broader too, where in the 1990s I listened almost exclusively to electronic music, I started getting back into Rock, and some of the new musical forms like Post Rock (a ghastly term, worse than "Progressive <anything>".
The cracks started to show in my taxonomy. Godspeed! You Black Emperor ended up in "Chilled", yet Tortoise and M83 ended up in "Rock", by virtue of using guitars. Goldfrapp is in "Chilled", despite mostly being stomping electro, but something you could go to sleep to. I dunno, it doesn't make much sense now.
My new toy has prompted me to revisit this issue. I've long avoided music library software, preferring to browse by my own folder structure, and I despise shuffle in most circumstances, either choosing individual tracks or listening to whole albums. The Squeezebox allows me to browse by my directory structure, but it also has a bunch of other useful and cool ways to slice and dice my music, so the time has come to get my music tagged properly.
It's going to be a long process, tagging all my music. I've generally gone for <artist> - <album>/<trackno>.<trackname> but with so many, for example, Bowie albums, that's getting unmanageable too. So I might end up with artist/album/ after all, though I will never do the brain damaged thing iTunes does and move stuff from compilations into individual artist directories -- that's just dumb.
So I'll shortly be starting the Great Retagging. Any suggestions on tools and approaches?