Audio jacks. I have a 5.1 system, and to use it properly I have to install HDAJackRetask. You can't just specify 5.1 surround sound from the distro's standard audio settings menu.
Pavuctl still is kind of mandatory to have with most DEs (you should be able to set the proper audio profile in it for 5.1?). Amusingly even on Pipewire systems.
Especially weird to see on Gnome. Audio Settings are rather meh, you still can't even set how the background is displayed since Gnome 3 either (centered, stretched, fit etc. - unless you know about the "optimizations" app) โฆbut look at our new "Wellbeing" feature! ๐ฆถ
Another is fstab. You'll often be told to go edit that by hand, often in the terminal. Adding a drive to an existing system and mounting it as part of the file system is a task an ordinary computer user would want to do.
I'm thinking about a gamer switching to Linux, and then saying something like "I wanna hook up my 5.1 speakers" or "I'm gonna buy a new SSD and add it to my existing system so I have room for more games."
I'm not a proponent of making EVERYTHING a GUI setting because not even Microsoft does that but there are still some splintery edges in places people will actually touch that could use some sanding.