'Flat' UI design is BAD design, plain and simple.
I've come to like the flatter look. How many bevels, gradients, shadows, and reflections do you need to show that a button is a button? In an Android app, a simple black and white icon is usually sufficient. Desktop apps may benefit from a little more polish, but the long term trend has been to use less and less "chrome" to indicate that a screen region is actionable. (If you're old enough, you'll remember how simple/lightweight hyperlinks were when you encountered them for the first time... just some blue text and an underline!)
If anything, these Windows 10 icons aren't flat and "iconic" enough. They should be aiming for something even simpler, like how the Visual Studio 2012/2013 designers used mostly simple silhouettes, with a handful of subdued colors for visual separation and emphasis. (The proposed Windows 10 icons are also just ugly. And what's up with turning the folder sideways?)
I think there's a parallel here to Edward Tufte's aesthetic that every piece of ink on a graph/chart must convey something meaningful about the data to the user. Any unnecessary element is chartjunk. Smartphones have forced us to apply the same thinking to UI design, and while some designers will inevitably take it too far (like Android did with the triangle/circle/square thing in KLP), we're headed in the right direction. Our chief scarcity is human attention, so it makes sense to generally de-empasize UI elements to the point where they don't ask for more of the user's attention then they deserve.