To my mind, collaborative playlists are one of the great successes of Spotify. You and your friends are going on a long road trip, so you make a playlist that you all contribute to. It is fantastic.
However, Spotify has decided that all collaborative playlists should be private. They did this because as we all know, anything that can be edited online will quickly be vandalized. The thing is, I would very much like to publish my collaborative playlists, because they are some of my favorites.
So this is what I propose. Create an option for collaborative playlists to be viewed publicly, but edited only by invitation. That way anyone can see the playlist, but only a selected few people can edit it. This is done on a whole host of other technologies, so why not do it on Spotify? Please spread the word and give this idea some kudos to ensure that it happens!