What makes a computer easy to use? The field of Human-Computer Interaction studies how humans and computers interact, combining the fields of computer science, linguistics, social sciences, and psychology. Much of the work in this area has the goal of simplifying complexity and making computer interfaces simple and intuitive.
A great introduction to this field is the work of Dr. Donald A. Norman, author of The Design of Everyday Things. In this book, Norman examines how ordinary objects are designed and explains how failures of a product are often failures of their design. He uses principles of cognitive psychology -- how people perceive, remember, think, speak, and solve problems -- to explore fundamental principles that guide good designs. (Notice the picture of the teapot on the cover of his book.) Another classic book by Norman, The Invisible Computer, goes so far as to argue that the computer ought to be hidden behind the scenes. His premise is that companies have become too technology-centric and need to focus instead on designing products that are human-centered or activity-centered. In this work he advocates for information appliances -- simple objects with computing embedded in them.
Norman's seminal works were published in the 1980s and 1990s, yet much of his philosophy speaks loudly to us today. How often have we encountered computer products that just did not seem to do what we wanted them to do? How often do companies focus on technology instead of people? Have you looked at a TV remote lately? Bad designs are everywhere.
We may be seeing the realization of Norman's information appliance dreams in the smart phone. The iPhone has a simple interface, without some of the complexity of a desktop computer. Applications are more task-focused, providing a basic todo list, note-taking abilities, calendaring, access to Facebook, or gaming. Most applications use a small set of standardized features. Are we reaching the point where the computer is becoming invisible and interfaces human-centered, or is the iPhone just another complex, general-purpose computing device in a smaller form factor?