The spacing of menubar icons makes it barely impossible to use tools parked in the menubar on a 13" MacBook without an external monitor. The fact that Bartender changed to not switch between two sets of icons, but more so expands the one to the left also doesn’t help. (Yes, I’m aware of the reduced spacing option in Bartender, but for whatever reason, it looks worse than before Big Sur.)
Dark mode has taken a step back and the border-gloss makes windows look even cheaper.
The rounded corners of all windows create weird artifacts in the corners of the display or when windows are tiled.
The fact that apps can colorize accents of the save dialog is distracting and cheaply executed. If I’m using MS Excel, for example, save a document and then resize the save dialog window there is a delay of the green accent overlay snapping to the blue base colored items of the favorites sidebar.
AirPods auto-switching doesn’t work 8 out of 10 times for me. And my AirPods get stuck in a connection attempt loop almost daily I have to revoke the pairing and repair them.
Connections via AFP to external network devices just randomly stop working.
Connections via SMBv3 sometimes get sluggish for no apparent reason and it takes ages to load a folder.
But even local folder discovery feels significantly slower from time to time (with sufficient free disk space).
And the cherries on top are the horrible icons in System Preferences that make it harder to parse the grid. This also applies to some other system icons, especially the Launchpad one.