My friends and I have created a shared playlist and I think this is a great idea and an awesome chance to differentiate Spotify from other music apps.
1) Implementing a simple voting scheme (a la Reddit) would allow for submissions to be deleted or sorted by global popularity.
2) Comments. A comment system would allow for each song to have a conversation of impressions and opinions that all could read or contribute to and would tuck away when not being viewed. Similar to an excel comment arrow.