The whole CLI. Linux should automatically generate default GUIs from manpages and code, to be developed further by the crowd of users on the desktop. It’s pointless to handcraft both interfaces one app at a time.
I like Linux Mint (compared to Ubuntu, Debian, and Windows) because usually right-clicking takes me closer to the solution I’m looking for, but it doesn’t allow me to dig deep enough. It should be discoverable all the way from the desktop to what makes it tick. Think of Smalltalk by Alan Kay in Xerox PARC in the 1970s, or what it would be now had it been mainstream all this time. #discoverability #explorability
You may be interested in some history of computer-aided collaboration: https://www.quora.com/Who-invented-the-modern-computer-look-and-feel/answer/Harri-K-Hiltunen