according to https://secure.wikimedia.org/wikipedia/en/wiki/Operating_system
An operating system (OS) is software, consisting of programs and data, that runs on computers and manages computer hardware resources and provides common services for efficient execution of various application software.
key words: "Provides common services for efficient execution of various application software." Let's be aware that we wouldn't be able to efficiently execute software without a terminal script (GNU's bash), coreutils to provide commands for humans to navigate computer and run programs (GNU's cat, ls, rm, chmod, cp, mkdir, cd, mv, printf, su, etc.), a compiler to convert high-level code of Linux kernel to various machine language (GNU's gcc), the ability to make common libraries and link them together with compiled object files to make executable files (GNU's ld), and many others key GNU components, at the least.
Examples of popular modern operating systems for personal computers are (in alphabetical order): GNU/Linux, Mac OS X, Microsoft Windows and Unix.
Apparently the long and complex Wikipedia decision making process has reached the verdict that GNU/Linux is indeed an operating system.
But yeah, I don't really care about Stallman's whining.