As a long time Mac user, in my opinion what has historically distinguished the Mac from other platforms is the high degree of UI/UX consistency across applications. Mac users historically valued software products that conformed to the Apple Human Interface Guidelines and tended to shun non-compliant products; this is why Microsoft Word 5.1 was so well-received by classic Mac users while Microsoft Word 6 was rejected by many of these users (
https://lowendmac.com/2013/microsoft-word-for-mac-faq/). While consistency among popular Mac apps isn't (and never has been) perfect, there's much more consistency among Mac apps than among Windows apps or among the X11 ecosystem (though, to be fair, consistency improves when sticking to apps designed for a specific desktop environment such as KDE or GNOME).
This is just my opinion, but I've found the classic Mac OS and pre-Yosemite Mac OS X Apple Human Interface Guidelines to be quite well thought of. In fact, while I prefer the technical underpinnings of Mac OS X to those of the classic Mac OS, I prefer the classic Mac OS's interface, which seems more intuitive to me.
Having the ability to theme one's desktop is nice, but I prefer having an ecosystem of well-designed applications that all conform to a common set of well-designed UI/UX guidelines. This is what has kept me on the Mac all these years, and my disappointment with the Linux desktop is that while it's easy for me to theme the desktop, the applications themselves are inconsistent, theming is only skin-deep, and what does not help is the fact that there is no single toolkit, desktop, and/or UI/UX guidelines (though, once again, I can achieve some greater consistency by sticking to Qt/KDE or GTK/GNOME apps). Granted, I admit the holy grail for me is for those well-designed applications to also have customizable UIs.
However, in an age where Electron apps are becoming more common and where M1 Macs can run some iOS apps, there is less cross-application consistency these days on the Mac. I regularly use Zoom and Slack, which clearly stand out when compared to applications that use the native AppKit API's controls.