For me the icing on the cake was segregating different apps between "tablet mode" and "desktop mode," where the former would take up the entire screen with no reasonable way to multitask between programs. Not only that but how many apps had (and even still have) both a UWP and standard implementation, from my experience that just confused users as to which they were supposed to use. Heck even today we still have the Settings app and the Control Panel (which is increasingly difficult to access I might add), both with various controls and settings only found in one but not the other.

IMO Windows 10 (sans tracking and forced pre/auto-installing apps) is what Windows 8 should have been from the beginning. Many beneficial changes under the hood, and minor UI changes that don't entirely change the workflow of the OS, but instead augment it for a better experience for both regular and power users. Somehow we've eclipsed the 5-year mark since its release, yet Windows 10 still feels incomplete. Not even just that, but they manage to make it feel even less finished with every new update.

Always amused me that the OS X Launchpad is also a ugly grid of huge app icons that takes up the entire screen, just like the Windows 8 start screen, yet nobody says a word about that.