I've been using Spotify for a long time now and have been participating in a collaborative playlist with three friends for over two years.
One function that would hugely improve our enjoyment of the collaborative playlist would be for us to be able to comment on individual songs in the playlist and start a critical discussion of the song's merits (between only the members of the collaborative playlist). Each song could have a drop down arrow which when clicked expands a collapsable list below it with the discussion thread.
I'm surprised that this function doesn't already exist. It would be great if you could add it, or otherwise let me know why this can;t be done.