I've always wanted a music recommendation system that worked in some sort of latent vector space. Song2Vec if you will. That way you could search for music similar to how people describe it. Something like: metal + Abba = Ghost.

I know Pandora used to (still?) was doing this, but the features were never exposed to users, and were very music theory. (Apparently I like vamping, but until I looked it up, I couldn't tell you want it was.)