For one, remember that Metro is deeply driven by touch devices. As a general approach, Microsoft's designers made the decision to avoid skeumorphism by making _almost everything touchable_. It's a great philosophy for touch interfaces, and it works quite well on today's windows phones.
Another piece of the puzzle driving metro is Kinect. Since Microsoft uses the same design language on all its platforms, the XBox shapes the desktop. Since the Kinect has trouble reading vertical hand gestures, the designers push for most desktop screens to lay out horizontally like the XBox.
With just those two small examples, you can see why the Metro desktop experience is both flat and horizontal (in many places, not all), which this author interpreted as thoughtless.
To reiterate my point, I think the Metro language is very well considered for devices that ask users to complete one task at a time (mobile, tablet, television). It's not so great in a multitasking environment, perhaps because Microsoft is trying to make their cross-platform experience too consistent.