Earlier quoted context omitted.
I find your comment unfair and I am not even a MS user. Usability is so much more about how it work, not just how it look. All design systems will have some learning curve. A learning curve where you have to understand that everything a button is pretty close to being easy to learn. So in my mind, the problem you seem to think there is is a pseudo problem. I.e. it's not actually a problem. The Map is not the territor…
The difference is that glossy buttons have affordances that make them stand out from the rest of the interface. Which means that even after learning what is and isn't a button, you can find them more quickly. Completely flat interfaces lose this advantage, but they also have other benefits.
Microsoft's problem isn't inherent in having a 'flat' design. It arises because of their alternately uniform (outlook), and seemingly arbitrary (metro desktop), color, contrast, layout and font-weight choices.
They could conceivably shift to a design-language where those attributes could be used to hint at interactivity at least as well as a big shiny buttons.