One of the reasons why I still use Windows 7 is that I like its 3D transparent glass-like look and utterly hate the flat design of later versions which I would call soulless and ugly. And the UI elements are harder to distinguish for me.
That being said, many people do like it, and I am not aware of any studies about how many people like it / don't like / don't care in general.
Fortunately for us flat UI haters, that horrible design trend will go away the same way it came - as a fashion. Fashion by definition does not last. Sooner or later flat design will inevitability be considered old and unhip too.
People used to design for desktop first, then port to mobile as an afterthought. Originally, there was only desktop, and mobile was a completely different and separate thing. Then mobile got more capable, and by necessity (since everything was designed for desktop) mobile tried to be a small desktop.
Mobile was limited, and trying to be a small desktop didn't work out that well. So people switched gears and said "Well, let's treat everything as mobile first, and let desktop adapt because it's more powerful." So we began the mobile-first trend.
But in the meantime mobile has gotten much more powerful and less limited; today many mobile devices have higher resolution and more powerful cpus than the old desktops had. The screens have also gotten a lot larger than they were when mobile-first started.
So, I think we'll either wind up with a "tablet-first" trend, designing for the middle-ground and making both the smaller mobile and larger desktop adapt, or we'll go back to recognizing that mobile, tablet, and desktop needs are different and we'll design accordingly for each, but now with a better understanding and better toolset for minimizing the redundancy that can occur with split designs.
On the other side of the spectrum, plenty focus too much on being neat and consistent that they become greyish, bland and sacrifice usability. Good ones use colours, shadows and placements wisely so that navigation does feel lighter and the experience is more content-focused.
Excellent ones do all that but without burdening the audience with that nasty aftertaste of pretentiousness.
Learn the rules, then break the ones you can't agree with. It's the designer's privilege to be able to do this more often than developers can ;)
I'd really love it if MS embraced shell skins. There are so many great ones, but I hesitate to jump through all the hoops to make them work.