When looking particularly at the evolution of MS Office, I often land on the view that UI peaked around 2010 and promptly shot down in ruins with the advent of flat design. It's flat design which went away from gradient textures on buttons which for preceding years familiarized us with the impression of button states, importance information, and a handful of other subtle things.
The stated reason was simple and understandable, the designs had needed to remain sensibly consistent when switching from small form factors to large ones, when resizing (or rotating e.g. when a tablet goes from horiz. to vertical, etc. But I'm not satisfied with this defense: it seems lazy, the gradient textures could have been retained. It would have required a lot more work by Microsoft/Google UI designers, but they chose not to step up to the plate by just giving it all up to go the easy road. At least Apple has had the courage to not take the easy way out.