But I don't get the complaint about multiple screens either. It's always been plug in -> desktop extended. I wouldn't even know what to get wrong.
There was always weirdness about the dock and menu bar that's sort of now settling down, but the dock used to display all sorts of bizarreness, especially when it was set to autohide.
Fundamentally it came down to issues with Apple's way of thinking about workspaces, monitors, desktops, applications and windows. Even now I still have to spend far too much time hunting down things across multiple desktops and monitors. It seemed like Apple was dancing around what should probably just happen in an effort not to too closely mimic Microsoft.
But to get at the heart of it, what happens when you full-screen an app? Apple had no idea what that meant when it was launched in 2011. However, what Apple chose to do in the case of full-screening an app on multiple monitors made absolutely no sense and was absolutely useless: the app would take on the role in the hierarchy of a desktop, and the other monitor would go grey. Why? nobody outside of Apple knows, but it took until 2013/2014 to fix.
Everything you need to know can be summed up in the fact that, in OSX, you can click the green button with a '+' in it, and that window can get smaller.