A chat box (kind of like what is used in Sound Drop) is practically essential in the collaborative playlists. The whole reason for sharing music is so you can enjoy and discuss it together. Let me know if this is possible! 🙂
Also the capabilities to comment on all people's playlists, songs, etc....but im sure thats in the works already...
Those ideas have already been suggested here:
http://community.spotify.com/t5/Spotify-Ideas/Group-chat-functionality-for-collaborative-playlists/i...
http://community.spotify.com/t5/Spotify-Ideas/Add-the-function-of-a-quot-newsfeed-guestbook-quot-to-...
Add your kudos there please! 😉