Many of us have a playlist like "Rock" or "Acoustic/Chilled", and we're all looking for a way to make it the best possible playlist for ourselves by discovering music.
If I have an acoustic playlist with 80 songs, and some random person (Bob) has an acoustic playlist with 49 songs, but 36 of these are in both of our playlists, then I'd love to know what Bob has in his playlist and he'd likewise like to know what's in mine.
I suggest a feature in settings "show suggestions for your playlists" which show up at the bottom of your playlist as greyed out - purely to make you aware of the songs, deleting them shouldn't be a problem, and if you like one or two, then add it to the playlist and more greyed out songs will then appear.
I know it will require a large amount of querying, and a heavy amount of hardware, but the results would be a fluid music discovery service which complements the information from spotify radio.
Sharing playlists is one thing, but music discovery is the most exciting thing about spotify in my opinion. iTunes has Genius - so get ahead of the game.
Please bear in mind that the Community is not an official Spotify support service. It's a place where we all help each other, whether we work for Spotify or not. So please use your discretion when using the forum.