It may be a very mundane complaint to many, but: The fact that I can't put the taskbar on the left side on the screen is reason alone to stick to Windows 10 for me. Also, the size of the taskbar.
On the desktop the main issue is that main taskbar can't be put on the secondary monitor anymore. That forces start menu and all the notifications appear over my full screen apps and games.
I assume they judged it was better for end user experience to make dumb assumptions always true rather than try and convince programmers to stop making dumb assumptions.