Basically how much more features it has and you miss in any other desktop. Especially in macOS, which we were forced to use at old company, I struggled with missing stuff and utterly stupid window manager. Yeah KWin window manager is especially nice in KDE Plasma. But also how much it has this LEGO-bricks like way of creating the desktop itself. All the desktop is basically constructed from small widgets and small widget containers which can behave in a very different way. So widgets (Plasmoids) are not only an addon to add to the wallpaper, they actually build up the entire desktop. And by this you can really make the desktop behave very close to what every individual person wants. Sure there are some presets that can emulate Windows or GNOME or macOS desktop, but yeah you can reconstruct that and have it your way. The similar story is there also with the apps GNOME/GTK versions usually just are to limited in functionality as far as my taste goes and I end up missing way too many.

P.S. I remembered another thing. Despite having more features and all KDE Plasma is also performing faster and is using less resources on the computers. So just another thing I like more.