If you're interested trainspotting individual CD serial numbers, then there's nothing quite like it. But my gut feeling is that there's a lot of folks with digital music collections who don't give a hoot about that stuff, and are interested in metadata about the song they just heard (the who, the when, the instruments / genres / moods, lyrics). For this audience, there are few decent, clean metadata sources. But when you ask, people will point you to MusicBrainz or tools that use it as a source (like Picard or Beets). You have to fight tooth and nail to avoid tagging a 70s album year as "2005" because there happened to be a remaster that year. Who. Fucking. Cares what year a Remaster happened when it's a 1976 album that you don't want showing up in your "Best of 2005" smart playlists.
As for social features, nobody is ever going to use MusicBrainz. They might have used RYM or Last.FM before the site owners made them unusable or pointless.
As for social features, nobody is ever going to use MusicBrainz. They might have used RYM or Last.FM before the site owners made them unusable or pointless.