birdie's question about 'grep' and djfiander's and Blake's replies reminded me of Thomas Scoville's article bearing the above title.
At the heart of Unix, Linux, and related languages is not the graphical user interface, but the command line, and the command line is a space for textual interaction with the computer. To master the command line, one learns a large number of commands, each with their own semantics and syntactic options. There is even a syntax for joining a string of commands to one another, such that the output of one command becomes the input of another. Thus, rather than presenting you with a vendor-defined GUI with a fixed combination of operations, the Unix command line lets you construct a myriad of unique "sentences" out its commands, each suited to a particular set of tasks.
This linguistic character of the operating system doubtless contributes greatly to its reputation for complexity, and not altogether without reason. It is complex, but its complexity, once mastered, also offers flexibility and power. I like to compare Unix/Linux servers to Windows NT servers by saying that Unix machines are like sailing ships, while NT machines are like motorboats. Looking at a sailing ship, you know you have to learn a great deal about rigging, sails, and winds in order to get anywhere with it. A motorboat encourages you to think you can just hop in, turn the key, and drive off, even if things really aren't quite that simple.
Scoville contends that, because Unix is language-centric rather than image-centric (as GUI-oriented OSes are), its devotees are disprortionately drawn from liberal arts backgrounds, and they are more likely to approach their work with a linguistic and literary mindset.
This article may seem a bit dated now that Linux is making a serious play for the desktop, with dozens of window and desktop managers to choose from, and with an increasing number of user-friendly desktop distributions. For those of us who cut our teeth on the command line, however, its text-oriented character remains an important strength of the Unix/Linux family of operating systems, and an essential part of our computing worldview.