I like to listen to jango.com which tries to "push" new artists while you listen to some "station"... I've found a few good "completely unknowns" there, but most stuff they push is really low quality. I think that a platform where you could discover new artists easily using a system similar to this (play mainstream artists, mostly, but throw in a few songs from unknown artists that play similar genres), but then had an option to "upgrade" to get stuff like offline access to artists you like, could become very successful.