Flat design is just design. It's a type of design like anything else; there's nothing "proper" about it. It's not "proper" when people screw it up because they don't include sufficient visual interface cues which result in a drastic drop in usability (I'm looking at you, Microsoft). It's just design. People can execute it horribly, and other people, like, say, Google, can execute it remarkably well.
What I find is that design is far too polarizing. There's no middle ground between skeuomorphic, three-dimensional, and flat interfaces. Why can't we have something like OS X 10.6, where there were elements of all design trends, and the result was a clean and very usable interface? Why did they have to fuck it up with 10.7 and the awful skeuomorphism? Why can't we have drop shadows? What is so "unproper" about them? Why can't we have gradients? Is it "unproper" to have those too?
This conversation has gone on long enough, and I'm tired of it. Flat design is just design.
End of story.