This wouldn't be such an issue if they provided an alternative. Or hell, make it themeable, even at a very basic* level.
And no, the Windows 95 style 'alternative' isn't reasonable.
For example, adding settings that were only:
* UI element bevel, inset: 0 - n pixels
* UI light source: Top Left, Top, Top Right
You could allow unflattening of UI elements with just some bevel drawing and light sources. Hell, it could be a normal map on the UI elements and the GPU could render the bevels itself, no line-drawing required.
Metro is a big ass of a UI, that requires the user to learn locations of functionality rather than being easy to discover by use of functional groupings and large identifiable icons (not small monochrome abstractions). It's one of those UIs that's good on paper, and maybe screenshots (because of the blocks of colour), but is just horrible to use.