Getting rid of the customisable/movable taskbar and replacing it with the god-awful Mac-style centre menu was an absolute travesty of design.
Similarly, replacing functional control panel dialogs with the "settings app".
The insistence on packaging programs as "app bundles".
Then there's the slew of useless always-online garbage, which literally nobody asked for, and which basically amounts to user-hostile spyware.
MS needs to accept that user interface design mostly peaked about a decade or two ago, and anything beyond that has been pointless tinkering at best, or actual regression of usability at worst.