What was/is the most annoying thing about Windows when you were/are using it?

Ctrl+c not copying on rare occasions. Even if it's my keyboard's fault, it could be avoided with some visual feedback to confirm to me that ctrl+c was registered ans clipboard was updated, so that I'd immediately know that it didn't work after pressing ctrl+c, rather than later when i switched to a different window/tab and pasted the wrong thing

The fact that i can't route audio between apps (without 3rd party closed-source apps). Why is something so basic not included into the system?

Registry

As a c++ dev: winapi. Right away you are greeted by windows.h adding loads of macros with common identifiers without any prefix to your preprocessor. That's a sign of things to come for anyone who has to use it. Maybe that explains lack of open-source audio routing apps: nobody wants to deal with windows driver development for hobby - and if that's the case i sure can't blame nobody for that.