Under the hood (not de's or gui) what REALLY separates linux from windows?

Is it just / ?

I kid. But really, besides "its all a file", if you take away the gui, is the only difference the syntax ? How libraries interact? How disks are mounted ?

If we stripped all ms's junk out and made windows open source, would we still prefer linux?

When you get to a very basic level, is one of them more efficiently coded?