My idea is basically what they already have on last.fm, you can compare your music taste with others and friends. But since Spotify is somewhat of a social media it would be cool to integrate a similar feature into Spotify and base your taste on favorites and, playlists and tracks you listened to more then twice.
Also it might be cool to have some kind of friend suggestion system combined with that so you might meet new people in your city with similar taste or get 'concert buddies' etc.
Although this idea has been suggested first, I'd like to point you to a similar, more popular idea here:
Add your kudos there please! 😉