You evidently don't actually use Windows 8.1. The much-maligned UI is actually just the Windows 7 UI with a full-screen Start menu, which I find interrupts my workflow to exactly the same extent that the Windows 7 Start menu does, meaning minimally.
Indeed I can live with the start screen. It's awkward, but I can live with it. The real disaster is elsewhere and I can't believe I still have to point this out after 2-3 years.
1. Default apps for many file formats are ridiculously dysfunctional Metro versions. This means users are cast into Full Screen Hell, showing Beelzebubs re-imagining of a PDF reader, image viewer or music app, designed for those confined in the darkest levels of hell. Escaping from these apps is actually hard. Noone can hear you scream.
2. Charms Bar on the right that pops up usually when I don't want it to. Which is always. Heh.
3. Some other bar on the left with any Metro apps that opened, usually without me wanting them to. What is that thing anyway and why is it there. Why is having two task/app switchers in a single OS ever a good idea? WTF Microsoft! W!T!F!?
4. Settings Schizophrenia. Where is that setting? Full-Screen-Hell-Mode or Control Panel? Or (gasp) BOTH? Oh My @#(&$ing GOD!
5. Installed Apps.. Where do they go? 8.0 Put everything and the kitchen sink in the start menu. 8.1. puts nothing in the start menu. Where are they? They're in a level below in the middle of a huge list of stuff. The only reasonable way to open an app is to search for it. So you better remember what it's called, Mom!
6. Search. I'm running out of expletives. It manages to open yet another full screen abomination in front of me when I'm looking for "Supplier Visit Notes 15Jan.docx", AND it starts finding stuff on the Internet.. What the hell MS!! You've messed up just about the most basic purpose of an OS user interface which is to let me store files, find them back and open them!
Anyway, you may feel less anger and pain about the above than I do but the point remains that Win 8's peculiarity (See, I can be nice too) isn't confined to having a start screen instead of a start menu. I guess I could have made that point in just a single line. ;)