Skeuomorphism
It was the loss of skeuomorphism in (Windows) UI design in general that was the beginning of the end, IMNSHO. Now everything looks flat and bland and unintuitive, UI elements are difficult to discern from everything else, as a result there's far too much empty space between them and interacting with the system is generally just a frustrating experience.
Add in the specific case of Windows from Win8 onwards, the window manager is a total buggy mess that constantly glitches out when it's trying to draw things, which makes the whole experience even worse.
WHAT WAS WRONG WITH THE WIN95-WIN2K DESIGN LANGUAGE?
Biting the hand that feeds IT
