Building a web site is easy, developing content is hard. So hard, in fact, that it's the number one reason for delayed or abandoned web projects.
I rely on the following metaphor to demonstrate the importance of content to my clients:
Building a web site is like building a house. I can lay down the foundation, throw up some walls and run the wiring. But it's not until you move in your own furniture that it's going to feel like a home. Your web site content - text, copy, graphics, images - is your furniture. It's what makes your site unique and sets you apart from your competitors.