Before things like PowerPoint were invented presentations were done with projectors and slide decks. Each slide was on a piece of film and had to be interested into the slide deck one by one and in the correct orientation. It seems that whoever put the slides in put multiple ones upside down.
For me it’s the removal and change of UI elements. There is still no built in way to move the task bar to the top or side of the screen and to get a useful right click menu back I have to go into the registry and change a value. There is also the whole thing where you are forced to use a Microsoft account with no option to use a local account instead.