The problem I think is, trying to remember what commands run on what System and OS. Sometimes I need to get the OS' manual out to remember what commands to enter . The problem starts when You get to OS/Systems that never been released in the West. Japanese and Soviet/Eastern Europe spring to mind.
The difference between Amiga, Dos & mac is well known. But then you start getting in the murky waters of early 80's Computing, Where Basic (2 billion different versions ), CP/M ( Also Almost 2 billion versions ) GEM, and other Propriety OS's where supreme.
As a example, The Difference of changing a Directory between Atari's A800 Dos and CP/M where Huge. I think Today's Kids have it way to easy.
Where to Start. I've started My Emu road way back in 1998/9. It started with A5200 Emulation on WIN95. Then a month later Got my first taste with MAME, If I remember Correctly it was with 0.33B (something). Hell I remember being almost to get the whole Mame Romset on a CD and have room to spare.