Consistent UI, simple and fast built-in apps for common tasks, good unix subsystem, fast file system, pretty text rendering, system libraries that make it fairly easy to write good native apps, etc.
macOS has all this, but it had all this for a while. There are so many good things in it. Preview.app is great -- it even lets you do simple PDF editing, while being fast, reliable, and easy to use. CoreGraphics does beautiful drawing, reasonably fast, leading e.g. to gnuplot having pretty antiailased graphs on macOS but not elsewhere (via Aquaterm -- this might be outdated information though, haven't used it in a while). The find pasteboard is clever and convenient, as are document icons -- and dragging them into Terminal even inserts the path to the document. There are so many well-thought-out details on macOS.
I don't care about iCloud integration, sync with iOS, Springboard, tabs in Finder or Maps, Siri, iMessage integration (I don't use an iPhone). The new Photos app doesn't honor mac system conventions. The Mac App Store is repulsive to me in its philosophy, and its implementation has been inept. I miss the HIG. Airpods are cool, but connecting them with my MBP works only haphazardly as you'd expect of regular old Bluetooth headphones, not as you'd expect of Apple hardware talking to Apple hardware.
I could go on for much longer, but I'll stop now :-)
Well, if you don't have an iPhone all the iCloud/Continuity/Handoff stuff is going to useless to you :)
But with regards to your other issues:
> The new Photos app doesn't honor mac system conventions.
This is being fixed. Photos eschewed Cocoa when it was first written and then realized this broke everything, so they're slowly adding these things back.
> The Mac App Store is repulsive to me in its philosophy, and its implementation has been inept.
I can't do anything about the philosophy, but there's a new App Store in macOS Mojave that's native as opposed to being webview-based.
> Airpods are cool, but connecting them with my MBP works only haphazardly as you'd expect of regular old Bluetooth headphones, not as you'd expect of Apple hardware talking to Apple hardware.
They're supposed to "just work", based on anecdota gathered from friends. Not sure why they don't for you.
I feel like UI is one of the things that shouldn't really be complained so much about. If you're talking about apps, then it's sort of hard to have a consistent UI when everybody's doing Electron apps or using other frameworks like it.
Sure, electron apps get this very wrong. App makers are free to write software that prioritizes something else than being a good Mac app. But Apple's own apps should be good Mac apps.
Almost all of the apps I use are consistent, largely because I stay away from Electron. This is feasible on macOS because there are almost always native alternatives for things.
What ways matter to you?