"Single Window UI" ?? I just opened dozens of new windows in Desktop. Reviewer is either incorrect or the oversight has been corrected.
"Double Desktop" - funny, I've noticed that Win8.1 uses far less memory than MacOS or Windows 7, whether I'm running the Desktop environment or not. Also, which of the two you use is entirely up to the user. You can even boot directly to the desktop if you want. I don't see how it's a cognitive overload if you pick the one that works best for you and stick with it.
"Flat style reduces discoverability"? Not necessarily. They've eliminated most of the things you cannot interact with. This example?
Image
EVERYTHING on that menu is clickable.
"Low information density" - This is not a bad thing. This calls attention to the information that is important, and allows you to scale the info you do see rather than confusing you and overloading you with information.
Live Tiles are new, and not everybody has figured out how best to use them - as the author stated, they are a double-edged sword, and when good, are VERY good.
He also disses the hidden UI elements, but I've been HIDING UI elements for YEARS so that I didn't have to stare at them all the time. Sure, it's something you need to learn to do at first, but it's something many people did ANYWAY, and I've had zero trouble with it.
I can't talk much about it's usefulness on a tablet, since the only touch UI I use is on my Nokia (where it works incredibly well).
I think perhaps the author was either a bit preemtive with his review, or just doesn't like any sort of change. In any case, I'm not so sure it qualifies as a "scientific study", since much of it is subjective and completely ignores factual information as I've pointed out.
In any case, it appears as though he has similar things to say about iOS.
http://www.nngroup.com/articles/ios-7/