The next "killer app" will be the one that allows a person to find new music that he or she would never have even thought of looking for, but will enjoy ... such an application would ignore GENRE and simply focus on THE INEFFABLE ESSENCE OF PERSONAL PREFERENCE.

It's called iRate Radio. I tried it briefly, but I discovered that I was mostly discovering a lot of music I didn't like. I guess I'm picky. The idea is sound, IMO, but the implemenation leaves something to be desired. Everything is lossily compressed, which is no good for regular listening. Additionally, it doesn't play higher-rated tracks more often; it just has a cutoff at a certain point (five out of ten, I think), where anything rated lower than that isn't played at all -- but a seven is played just as often as a ten, and so you can't rate tracks you only sort-of like at six, because they play constantly and drive you mad. This forces you to rate everything you don't want to hear very often at five or lower. But the idea of ignoring genre and choosing tracks for you based on finding people who rated other tracks similarly to the way you rated them is an interesting idea, in principle, and could potentially be really useful.