Perhaps the solution is more holistic. A great setlist takes into account when a song was played last, its popularity, how it relates to friends’ musical tastes, and similarity to music that’s currently playing. Rdio could compute a compound index, Play Strength, and expose it in the interface to help suggest the next tracks. You don’t have to explain how you computed the index, but the mechanics are interesting. Pandora does this when they explain why you’re hearing a particular song and it adds to the experience.
We need more people like Avand working in places like Spotify and Rdio. Great music curation is a huge design challenge.