... And this is what worries me about much flat design: With one very good designer it is probalby all fine. But if many people design slightly incoherently with it and/or make minor mistakes it gets harder to use.... So it is not that flat design is wrong, it is very brittle.
I can certainly see how this makes a UI less easily learnable.
I strongly think that most, if not all applications should be easily learnable. Partly because everyone begins at some point and because needing to rely on learned things still will have some background cognitive load and proneness to errors.
For better or worse, most of us are pretty use to weak/absent signifiers in a mobile context by now--think about all the functionality on your phone that is only accessible through multitouch gestures, which usually aren't called out in the UI at all.