Something Spotify is missing is some sort of app or a part of spotify itself that allows users to see how similar their friends musical taste is to theirs. An app that could show this data, and also sort that list of friends by how compatible they are with you or other people would be a great innovation to Spotify. As it is now, the only way to gauge this is to actually look at what songs their friends listen to.
Changed back to "new idea" status - once this idea reaches 40+ kudos the Spotify staff will review it again and decide on a new status. ;)