Here's the thing...
Our brains are extremely powerful at identifying objects in real life. For example, if we look at a room with lots of small objects in it, we have no problem identifying each and seeing where one ends and the other starts.
Now, the early UI designers recognized this phenomenon and used it to their advantage. Gradients, shadows etc were used so that with just one quick look, you knew what exactly you were looking at, what can be interacted with, where one ends and the other starts etc.
To be very clear (and this can't be stressed enough)... This has absolutely nothing to do with skeuomorphism. They were not trying to make it look like anything in real life, they simply recognized that they can use that ability that every human being possesses to the advantage of everyone.
Now, on the other hand the geniuses that designed the "modern" language thought that removing each and every depth cue was a great way to "clean up" the UI. You know, make it just like the phone UI, where you don't need any depth cues at all because the UI is simple and nothing overlaps.
For anybody that understands this basic UI concept, having to be subjected to W8/10 is a giant step backwards because we know that what we are forced to look at was born out of pure ignorance.
This is why I now have trouble finding the thumbs on scrollbars (once you can even get them to show up). Trouble knowing which part is the thumb and which part is the track if it is scrolled all the way to the top or bottom, because the dark/light is reversed depending on which app/application you work with. Trouble identifying where one window starts and another ends when I have many overlapping windows. Trouble finding the title bar to drag a window with many overlapping windows. The hit area of the sizing cursor is now outside the window because they had to extend it due to the too-narrow border. Oversized controls wasting huge amounts of space, because placing them too close together will make it hard to visually separate each since all we are looking at are monochrome lines, rectangles and circles. Icons that lost their individuality and hence are now harder to memorize and identify.
All of this because someone had a stick up their @$$ about removing anything and everything that might possibly be misconstrued as being skeuomorphic on any level.
There is a good balance between being actually skeuomorphic (e.g. like Apple with their brushed metal, wood, leather), and having everything 100% flat. What we have now is a step backwards, and if you can't see why, it means you don't grasp these concepts. Just like the W8/10 designers, who should have been kept far away from being able to make these sorts of changes instead of letting the inexperienced UI designers experiment with something as important as this.