Explaining Complexity in Software Development?

BMazurek asks: "I'm stumped by how to explain software development complexity (not theoretical big-O notation, that's easy) to non-developers. When it comes to people who aren't in the code, my explanations fall flat. It's not that the people I'm talking to are stupid, they're quite honestly people at the top of their respective (non-tech) fields. How do -you- explain software development complexity to non-developers? What analogies do you use?" "I often try the famous Fred Brooks, Jr. quote (seldom to much success):

'Software entities are more complex for their size than perhaps any other human construct because no two parts are alike (at least above the statement level). If they are, we make the two similar parts into a subroutine--open or closed. In this respect, software systems differ profoundly from computers, buildings, or automobiles, where repeated elements abound.'