Comments on songs are not currently supported by Bandcamp, but I think their implementation of comments make sense: users can recommend songs at the album level rather than per song, so that all rave and rants about a particular album's contents can be found together rather than pieced apart in each song's download page. SoundCloud chose to go to an even more granular level and allow comments to be placed within a track's duration, and then also comments below each track. Both are interesting methods of offering a community.

Another form of community-building is what Beatport did, which encouraged remixes by offering stems and integrating with software that the artists used.

Another indie music site is good news; I remember fondly finding some good music on Amie Street before Amazon gobbled them up.