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.
Of course they are without any data privacy laws companies are going to collect and then sell as much of your personal data that they can get away with.