One thing seems abundantly clear:
Apple's HIG and related design principles have shifted, from pre-iOS concerns about ensuring that interfaces are so simple and obvious that anyone and their dog could use them without any learning curve (relatively), to post-iOS concerns about designing "computer" or rather "device" interfaces that are very similar in form and function to the appliances we use every day as a matter of course: iOS and Android devices.
From a pre-iOS perspective the UI design changes in Yosemite aren't very thoughtful from a new-user perspective - that is, the "I don't know about all that computer and phone stuff" perspective that is gradually fading away. There's much less UI hand-holding. But from the perspective of those by now accustomed to iOS and Android devices, most of it will look familiar, and in some ways we no longer require clearly-defined buttons or "depth" in order to help us distinguish certain elements from one another. Folks are just more technologically aware and know what to expect.
The only little niggle in all this, is that some of these iOS carry-over elements look a little out of place at this stage. They look natural on a handheld device, but look rather wonky in more places than not on a "computer" display. I imagine that Apple will naturally clean all this up for Release.
Lots of words to justify that a button does not look like a button anymore. While I do see your point, I do not agree this has yet happened with the iOS 6-7 shift. After iOS 7 was released I also read these enthusiastic posts about this new GUI paradigm and how great new user interfaces would emerge (flat screens, depth due to transparency, parallax, etc). Well, it never happened. Every single iOS 7 updated app just turned flat and white. End of story.
iOS 7 is more difficult to use and far less intuitive but it's just for esthetic purposes . It's fresh, clean and there's more room for content as lots of UI elements are hidden. But there's nothing you can do because of this new interface you absolutely couldn't do in iOS 6.