Personally, I find music discovery a very interesting problem. So many variables to play with! If user listened to artist X, should we recommend more of it or would it be too much? How important are recently played songs vs long term trends in listening (e.g. listening to genre X most of the time over the last year, however, this week I'm listening to Y only)?

Unfortunately, it requires to be running at spotify-like scale to do something useful, have a large enough library and gather feedback quickly enough :( oh well. I will continue dreaming about it though.