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?
They come from completely different heritages.
GNU/Linux is a reimplementation of Unix, an operating system that was originally designed mainly for universities, but also mainframes.
Windows is descended from DOS, an operating system intended for home computers.
Nowadays Windows is the only widely used non-Unix-like OS; GNU/Linux, Android, macOS and iOS are all Unix-like.
If Windows became FOSS, I at least would likely switch to it. It’s really the FOSS philosophy more than anything else that makes me want to use GNU/Linux.