The most difficult "upgrade" to swallow is Safari. If you have more than one tab (which is common), the UI uses 3 times more space than before.
I went back to Catalina and couldn't be happier. Don't know what to do when I buy an M1 Mac.
I dislike how the "new tab" button, which used to be to the right of the tabs, is now a title bar button. It takes some retraining to get accustomed to, but there was no reason to move it in the first place.
I hate how hard it is to move the Safari window around in Safari 14. This is due to the title bar being so cluttered now, making it hard to find a spot to grab the window by. I routinely work in two windows, one pinned to the left side of the screen and one to the right.
Better Touch Tool helps with pinning the windows (and for some reason that app started working again under Big Sur, when it stopped working on a previous update), but moving the window in the first place is now needlessly difficult.