In the past MS used http://iconfactory.com/
They did not use internal staff.
But the managers that approve it are to go first.
At least the folks at Icon Factory know a thing or two about iconography, which is as much of an exact science as UI design ever was; part pixel art, part language. As other 'dotters here have happily provided links to not only the historical iconography of Microsoft, but other platforms as well, you can see the evolution of aesthetic choices; the playful isometric simplicity of BeOS, the monochromatic elegance of NeXT, and the neo-realism of Gnome. Saying that the flat colors is a throwback to the primitive computer era (8/16 bit) is rather ignorant, simply because the color-palette choice wasn't a matter of preference, as much as necessity. Back then, the engineers were put in charge of defining the color gamut based on just 16 or 256 'slots' to use. Naturally, the engineers approached this in an algorithmic fashion, rather than aesthetically. That's why it took us 30 years to come up with color rendering that could represent natural/earth/skin tones, because there were all these mathematical gaps in the subtle spectra of blues, browns and greens. In that sense, I suppose the selection of flat saturated colors is indeed ironic in the age of hyper-realistic imagery. I applaud an aesthetic choice for elegant iconography, however the execution can be equally delightful or disastrous.
While I agree in part with the dissent over the design choices, I don't agree that TFA is representative of any significant "majority". Let's be real here, the headline reads, "icons look like a bad joke." Do you really think that contributing readers would be unbiased? You might as well have a big sign out front, "MS-bashing Trolls Welcome!"
But here's the catch. It's hard to have a serious discussion about UI choices even in this forum, one that's so inclined to conflate the design with every poor PR move, questionable politics and troubled past of the legacy platform, all making it impossible to take a step back and appreciate the design choices for what they are. It's also important to add that UI choices aren't just about making it artful, but mostly, meaningful. These mini-pictures are purpose-made to fall into the background, rather than be their own eye-candy. (that's what custom icon sets are for)
So here, I'll take a stab at it. This icon gallery clearly perpetuates the traditional Windows brand "manila folder" trope as a foundation. With flat colors and angled lines, it does an attempt at three-dimensional appearance, which arguably does look very 'flat', with or without comparison to its predecessors. While those do not make up 100% of the new icon set, the "folders" establish the overall paradigm and 'look' of the interface. I'm not convinced that the non-folder icons are even complete, since most of them still resemble Aero's photo-realistic set of devices. The icons that notably reflect the new art style are the "My Computer" and "Network" icons, which is a simple line-art treatment style. This is not consistent with the folder paradigm, not only because they don't resemble folders, but because these images are using boundary lines to define shapes, rather than flat colors. Overall, it's rather inelegant and poorly executed. The folders use subtle boundary lines, but inconsistently, and the line doesn't diminish on the smaller icons, making the left face of the folder look awkward, like a backwards "L" from a varsity jacket. Again, we see that the Redmond workshop has neglected the beauty of scale and only centers their model on an 'ideal' size, whatever size that may be, and also belies an underlying framework that is—yet again—bullishly ignorant of modern, precision rendering. As I'm running Win10-TP myself, I can also see that File Explorer attempts to express folder contents as foreground icons using the open-folder trope as a background. It renders another closed-folder icon atop the background if a sub-folder is present. A poor choice, since the flat colors run together and make the background folder look strange. We also see that, when the contents throw the object-recognition algorithm for a loop, the second foreground icon is a square outline; a tell-tale indication that MS still hasn't learned it's lesson about meaningful representations. If this was an in-house job, then I strongly recommend outsourcing it to an expert group once again, all the way down to the visual rendering engine. Aero was dumped, rather than evolved; another poor choice.
And there you have it; an attempt at a serious discussion about the merits and properties of the new icons that isn't just a splattered statement of subjectivity. I welcome anyone to contribute, and I hope (against hope) that the sincerity of this discussion may be preserved.