One of the big reasons I have always like CDE/Motif was that it, to me, is the epitome of function over form when it comes to a desktop UI and widget toolkit. It's actually the same reason I think the Windows' UI peaked around Windows 98SE. Things that are interactive, like buttons or window frame edges, look interactive and are easy to spot. Buttons look like buttons. Checkboxes look like checkboxes. Contrasting colors are used to separate window frames from window contents. Window frames had actual titles so that you could glance quickly to see the state of your system. You sat down with it, you knew immediately what was what.

NextSTEP/OpenSTEP/GnuSTEP feels similar to me. Yes, by modern standards, they are ugly as sin. But they were super, super usable for the users with very little ramp-up time.

By contrast, everything else is just so flat now. On my Mac, it's a regular Where's Waldo to figure out what is and isn't clickable sometimes. There is very little contrast between the window frames and window content ... if there is any at all, and I really dislike how they've largely done away with title bars entirely. And so much stuff is hidden in menus or behind obtuse, difficult to reach settings. I'll occasionally go look at Windows and it's not much better.

What's really frustrating is watching some of my elderly family members struggle to use newer versions of Windows or macOS because the UX has become so flat that using it can be very obtuse if it's not something that you use all day, every day. It feels like redesigning UIs is becoming a vanity project for companies to show they are "modern" ... rather than trying to make things better for actual human beings that use computers.