Two things the 1990s got right and (in my opinion) have never really been replaced: Pointcast and Moodlogic.
Moodlogic worked on my library, I could choose any song, then it would craft a variable length playlist based on that song's mood and instruments. There was a crowd-sourced set of metadata, and I was able to fill in the gaps or correct anything I didn't agree with.
If anyone knows of anything along these lines, I'd be grateful to hear about it.