The thing that really hit me about the screenshot was how crowded it looks. The example is presenting information with a clear underlying structure (a file system) and a small number of actions I can take, and probably half the area of that window is empty space. And yet, my immediate reaction is that there's no clear structure to tell me where to look, and the design desperately needs more visual hierarchy and better use of whitespace. Of course, this is a recurring problem with the current trend for flat designs
I agree that the screenshot looks more complicated than it needs to, but I'm not sure it's a problem with the "flat" graphical style so much as the layout which (IMHO) looks like versions of Windows from the not-at-all-flat Vista onwards (and even XP to some extent until you turned some of the crap off).
The problems with the icons there are- if anything- that they've moved *away* from flat design which (done well) would- and should- have simplified them to their essential elements and made them recognisable at a distance (à la road signs, etc.).
But, as stated by others elsewhere, MS has always been about change for the sake of change, playing silly b*****s by introducing new technologies and ways of doing things that are discarded in the next version of Windows simply for the sake of being new, or at least for selling some "new" crap.