The idea was a vast plugin ecosystem which would allow development of desktop features to be more distributed. The problem was, some of the most basic features (like a taskbar for displaying open windows, or panels, or applets, or sane window management tools) were left out.
At some point it becomes a security and performance nightmare when the idea is that to achieve a normal desktop experience, you need dozens of user-supplied scripts.
Of course that's not a problem in practice, because even with user plugins, several basic features continue to be missing. And the fact that most of these plugins are just recreating functionality that already previously existed should really tell you something about GNOME team's design philosophy.
To quote Linus Torvalds, GNOME 3 is a "total user experience design failure".
From https://www.phoronix.com/news/MTMxNj:
> He says that GNOME 3.x has been improving and that when pairing the desktop with the right extensions, it's a rather pleasant desktop. He specifically points out Frippery and GNOME Tweak Tool as making a huge difference to his experience.