The Linux desktop is flexible and customizable. I wish mainstream apps would run on Linux as well, so we could experiment and further develop established UI paradigms.
Except it isn't. In order to deal with something as simple as unifying how keyboard shortcuts work across the UI (the desktop environment, intra-UI idioms for movement & selection, etc.) I'd have to unpack 30 years of legacy X11 decisions and idiosyncrasies, sort out how to reform numerous different UI toolkits, and often dig into the applications themselves on an application by application basis.
It's often impenetrable despite its being open source.
It's often impenetrable despite its being open source.