I've been using Sonoma now for just a few days now, but there are a few things that I'm already impressed by.
Wallpapers
The first being the new animated wallpapers, and how they transition from the Lock Screen to the desktop. It's such an Apple feature, which I don't feel like we've had for a while. The Lock Screen having an animated wallpaper would be great on it's own, but the fact that it slows down, and then settles on a frame is 10x better.
(I've just been going between the various Scotland options so far.)
Web Apps in the Dock
Then I've got to mention having web apps in the Dock. I've already got 2 configured on my personal machine, Fosstodon, and Chess.com. And I've also set a few up on my work machine, for common web apps that I use, like, for example, our document management tool. Sometimes websites don't have apps, and sometimes the website is just better than the app, so I think I'm going to end up using these a lot.
Another bonus for this feature, is that if you view a website that you have added to your Dock, in Safari, it will prompt you to open it in the app instead.
Desktop Widgets
Finally, there's Desktop Widgets. These are so much more useful than having it hidden away in a menu.
I've currently got just Weather, Calendar, and NetNewsWire widgets at the moment. I'm not sure if I'll ever have a ton, but just having these few details in a place where I'll see them often will be very useful.
These aren't exactly groundbreaking features. But I don't particularly want the Mac to keep changing. So these little touches go a long way for me.