Gnome's polish is probably the best of the Linux DE's but it removes far too much configurability and features (in the name of simplification) for my tastes. I'll stick with KDE. Each release gets more and more polished, and I'm very much looking forward to Plasma 6 and the Wayland Goodness it will bring.
Having said that, I think the competition between them makes both of them better so I'm glad that both exist. And where they depend on common libraries, both benefit once again from the contributions from the other's contributions.
Although I do prefer look and feel of the GNOME and most of the software I use is GTK, I use Plasma as well. The main issue I personally have with GNOME is it's dependency on 3rd-party extensions. For start, it's missing clipboard manager and notification tray that you get with Plasma out of the box (as well with other DEs and OSes). You want the dock? Again, you need the extension. Then you upgrade and at least one of the extensions is lagging behind.
No kidding. If I can get same battery life as macOS on my MacBook, trackpad gestures, OS-wide Emacs-style keyboard shortcuts and Microsoft office I'm not going back. Proton has made it possible to run previously Windows-only games.
There are numerous examples on freedesktop.org. Right near the top of the page you've got things like D-Bus and NetworkManager in there. Then there is FreeType, Wayland, and so on.
Having said that, I think the competition between them makes both of them better so I'm glad that both exist. And where they depend on common libraries, both benefit once again from the contributions from the other's contributions.