I always like it when someone gives a talk, attaches his windows laptop to the beamer, and is then constantly distracted by the yellow notifications in the bottom right corner "You are now connected to the internet", "Your virus definitions are out of date", "New hardware detected"...
Of course, OS X is not as clean and notification-free as I'd like it to be: It annoys me every other week or so with some useless software update, and every 10 days it complains that I didn't plug in the backup drive for some time.
Now, it might be possible to configure and disable all these notifications, but I just don't want to bother. I want my computer to act on my command, and in my eyes there is almost no excuse for putting up an unexpected notification or alert or dialog for anything.
EDIT: is -> is (often)
While the OS doesn't necessarily have to distribute a graphical environment, it can and it is often a feature of the OS, unlike what rbanffy said.
I've edited my post above.
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.
OS space
Again, that's kernel space. OS is something bigger. Debian is an OS. GNU/Linux is an OS. Linux by itself isn't.
I must be mistaken.