I’ve also been living with Big Sur on my primary Mac for several weeks and that experience has been a little less delightful, and in some ways more of an adjustment than the smaller phone.
Just to get this out of the way: Bugs aside, overall I actually really like the look, and while I have some minor quibbles I expect that most of those will get tweaked over time, just as iOS did post-iOS 7. But…
The slippery notifications have serious usability problems that I really hope will get rethought sooner rather than later. The toolbars have completely broken my muscle memory, especially for Mail. As someone who often uses search in Mail sometimes dozens of times a day, the narrower available space causing the search field to be hidden/collapsed except when the window is much wider than I would normally keep it is irritating. The centred text in narrow dialog boxes is terrible.
I get why you might want that orientation as the default for an unoptimized Catalyst app, but it should not be the system default. I’d be satisfied if it’s possible to override that behaviour so that developers who care can adjust accordingly - some (many) won’t but that’s nothing new, Mac users are no stranger to apps with varying degrees of Mac-ness - but I hope the default appearance changes.
All of this together has given me the feeling that the OS - and the layout of many apps in particular - is increasingly designed for smaller screens, as in smaller than the iMac. I don’t blame Catalyst for this, as this is not actually new to Big Sur nor even particularly surprising - for a long time the vast majority of Macs sold have been laptops, and it makes sense for layouts to be most optimized for those screen sizes.
But more of my apps feel just a little out of place on my 27” screen now, and that there are some missed opportunities to take advantage of that space even if only the dialogs have changed in a particular app. One of the great things about the Mac is the ability - laptop or desktop - to connect screens of any size. Maybe there is usage data beyond the dominance of portables to support that direction, and I know designing layouts flexible enough to take good advantage of a wide range of screen sizes can be challenging. Whatever the reasons, I hope that a little more attention is paid to actually using all this screen real estate other than to just give elements on screen a little more breathing room and more generous spacing. After all, it would also benefit any larger iPadOS devices (or better external display support) in the future, should they ever materialize.
Or maybe it’s just me, and I just need to suck it up and get used to it*.
*except for those dialog boxes - I don’t think I’ll ever really get used to them.