1. Disabling and/or refusing updates.
2. Setting the font for the user interface. Locked at Segoe UI (if not using tricks).
3. Aero has been removed.
4. Using colors is (mostly) removed
5. Windows 8.x removed drop shadows, making the OS as flat as paper, which makes it almost unworkable for me (Windows 10 re-introduced drop shadows again)
6. Menu removed, replaced by the ribbon, which can't have it's font sizes changed and which changes layout when the window resizes.
7. Classic mode is removed. Some people like that mode, or actually require it because one could set colors for almost every element.
8. Internet Explorer is dropped and stated to be legacy in favor of Edge. Internet Explorer has a LOT more options though.
9. Removed privacy. (Or more accurately: Added large amount of privacy-infringing stuff when using the default settings. Even disabling everything doesn't disable all of the data transmission.)
10.
More (Windows 8),
and even more (Windows 10)