Information architecture of music is what Echonest was all about, and I'm pretty sure they keep doing what they were doing when Spotify bought them.
To the degree that judging good jazz or classical is different from judging other types of music, I think that yes, it's based a lot on opinion. In particular the opinion of authorities - critics and other performers.
It's not that this is entirely unreasonable. With music as a social phenomenon, you might prefer to not be "into" the wrong kind of music, even if you would like it for the music itself. Spotify and Echonest have actually talked a bit about how listening patterns can reveal "shameful" tastes, different from the tastes we would like to project.
The job of a recommendation system then, if we should look cynically at it, is to show you the "right" kinds of music that you would like to like, but actually like too - and to not tempt you with "wrong" kinds of music that you would like despite yourself.
And yes, I'd bet you'd need musicologists (or human analysts) for that. It would by definition be very hard to figure out from listening patterns or acoustic features. But isn't this a bit cynical as I said? Shouldn't we try to not be ashamed of what we actually like?
