I came across an article
that was discussing the trend against skeuomorphism, or using physical objects to represent something in a computer. For instance, a knob to turn volume, a picture of a notepad for... well a notepad. Or at least something that looks like you could go to Target and buy a notepad. Instead, UI designers are going towards flatter, cleaner elements.
One of the complaints I hear about Metro's UI is the flatness, and the choice of colors, among other things. That Aero is a great UI design that shouldn't be touched and Metro (as in the design language itself, we can ignore the apps and tiles) ruined Windows UI. But there's some issues with it. An example I go to from a useability standpoint is that it's hard to get an at-a-glance look at what window you have focus on. Another is that Aero is a resource hog, compared to other UIs, and its using those resources strictly for eye candy. In its first cut, Desktop Window Manager would happily eat up RAM if you had too many windows open. Finally, it's not very scalable... Which leads to one of the plausible reasons why there's a trend in "flatness"
Flat, simple UI elements can be made with vector graphics. If you look at the many icons and buttons in Windows 7, they're all bitmaps. Bitmaps don't scale very well to higher resolutions. It's like bumping up the zoom in a web browser (which is also struggling to keep up with the high DPI world). Also, going with simpler designs lets users focus more on the content of the window, rather than on the chrome. In fact, allegedly Microsoft said Aero was supposed to allow users to focus more on the content and less on the chrome (presumably because nobody pays attention to glass) ... which I'm sure most users did the exact opposite once they got Windows Vista or 7.
However, like the article suggests, we shouldn't get rid of the notion that the UI is a sort of 3D one, and should still have clear depth cues. Drop shadows under windows for instance, is an "eye candy" effect that actually enhances the presentation. I've also read up on Google's Material Design documentation. While they're also going to a trend of "flatness", a lot of it is anything but. But we should be careful in including elements that aren't necessary.
But then there's a problem that some users would rather keep 96DPI and go with 40" 4K monitors than 27" 4K monitors and not have to bump up the PPI... which they may think reduces spatial resolution. But the funny thing is, in order to enjoy a large monitor without having to turn your head everywhere, which by the way is an ergonomic problem, you have to sit far from the monitor. And then your effective resolution that you perceive is still no better than a 27" monitor of perhaps even lesser resolution. I'd rather take 200PPI and scale everything to look nice and clean on a 27" monitor than a 40" 4K monitor.