I considered both Windows 8.1 and Windows 10 long and hard last night after doing a rebuild of my (previously dead) main computer, but gave up and settled on sticking with Windows 7 in the end, as W8.1 is still a bit too touchscreen-focused (even with Classic Shell, which is a workaround that I'd rather not need in the first place) and W10 has an EULA that I won't accept. I'm not sure I'd say that they've outright "screwed up" with W8.1/W10, but neither of them are what I'm looking for as a desktop user.
PS: Oh, and the ads, of course. I'm not paying Microsoft to get served personalized spam built right into my operating system. It's possible to disable them, but that's again something that I shouldn't have to do out of the box, just like with Classic Shell.