A feature like this would make Spotify even more social. The foundation for such a feature is already in place. I can't say how easy it would be to introduce a feature like this because I'm not a programmer, but hear me out.
Spotify already identifies which songs you have "Saved" to "Your Music", and these are marked when viewing your friend's playlists.
If you could filter out the unmarked songs, it would be easier to find specific tunes that you both enjoy.
Down the road, you may even be able to compare "Your Music" to "[Your Friend's] Music". To avoid embarrassment over explicit songs, you could limit what your friends see to songs that you both have "Saved", via Privacy Settings. It would be even greater if you could compare music between three or more people at once.
Ultimately, such a feature would help you to easily find out what kind of music to play for new friends, or when you just want to play something for everybody. If you're like me, you have a hard time choosing music to play for your friends, and something like this would help a lot!