It would be useful to have some additional collaberation features, specifically the ability for the author to have a list of all changes made my users, and roll the playlist back to a certain date/time if its been changed, or tracks were maliciously deleted. It would save constantly having an additonal 'Backup' playlist in my sidebar.
The ability to block users, approve tracks or only allow certain friends to edit the playlist while it still being availible for the publicly would also be VERY useful, however I think that has already been suggested a few times. It would be nice if you could manually credit someone else with adding a track (say if it was removed/re added to a different playlist) which could then it comes up as a notification for the user. Similar to when you are tagged in a photo on Facebook.
Collaborative playlists are a great featue, but they are very basic and haven't changed much/at all since they were first implemented. If you search the web people have been requesting things like this for years!