As you build up playlists, suddenly they are full of music that you like, but are very different.
So, you can be all pumped up with a rock tune and then suddenly shuffles to a mellow mood kind of tune. It would be great to have a shuffle mode that follows the mood of the current song. Basically adapting the "similar music" functionality to make it work within a playlist. You can even throw in some new stuff automatically. Similar to Last.fm's Mix stations, but not quite.