Great answers already, I’ll not repeat them. One thing I want to mention though is the interoperability of the Linux applications. Things work together well. With Windows (up to 10 at least, I haven’t used windows much in the last years) applications are mostly their own silo. In KDE it’s quite fluent. E.g. gwenview, the image viewer offers to open an image in krita, gimp, etc. It also offers an option to add a folder to the “places” list in dolphin (the file manager). Dolphin lets you quickly (F4) open and close a terminal at the current folder within its window. Small things like these make the system feel coherent.
The other big thing for me is the plethora of great apps you have out of the box. And the ease to install new ones without worrying whether you are the product.