The new design language is more usable, and more attractive in my opinion. Something Apple has ALWAYS strived for. Attractive, usable interface design. The weightier fonts are much easier on the eyes and less bland. I personally think the new Music app is a big step up from prior apps. Certainly a massive improvement over iOS 9, which was failed style over usability. They tried to stylize it too much, but it wasn't even attractive. This new design language seems much more focused and knows what it wants to be.
As with all interface refinements, they'll keep iterating, hone in on it, and then I'm sure the HIG and UIKit will be updated to reflect the new style. It's clear that at this point, the new design language doesn't work for every app and isn't intended for every app.