You are confusing OS with the userland. Graphical environment is not necessarily a part of the OS. If you run Linux you can load a number of different graphical environments.
> If I use Linux I can load a number of different filesystems. Are these not part of the OS?
Actually, most filesystems are part of the kernel. There is the FUSE mechanism, on which you can attach user-space file systems. Filesystem drivers attached in such way are usually (unless you are doing something wrong) portable across OSs that share similar mechanisms and, as the name implies, are withing the user space. not OS-space.
> it is often a feature of the OS
Much like tires are a feature of a car. The fact you bought them together does not mean they are the same object. A tire is not a car much like a GUI is not an OS.
