Earlier quoted context omitted.
I think it takes a lot of work to ensure that removing depth and shading doesn't remove information, too. I think flat design inherently "wants" to be, if not user hostile, at least less user-friendly than what preceded it. It's a win if the depth and shading was just noise, before. If it was signal, well, now you've got to find something to replace that with, or you're harming UX. And you've got less "bandwidth", if…
Exactly. Flat design removes signals in order to let the UI appear simpler. But the removal of meaningful signals makes the UI more more ambiguous and harder to use. Flat design is an obfuscation, trying to cover up the actual complexity behind a visual semblance of simplicity.
Option this, Ctrl that. Yes, they can be discovered socially and informationally, but that goes against intuitiveness.