The difference between "design what the customer needs and no more" and over design is clear and also doesn't need explaining.

To clarify, I'm not an unthinking follower of "design what the customer needs and no more"; I feel that's a dangerous over-simplification. While writing code that is never used is certainly waste, and one has to beware of over-engineering, I don't view this as a black and white issue. I touched on this in the "Agile Design" section where I stated:

Software design is an art requiring experience, talent, good taste, and deep domain, computer science and software usability knowledge. I feel there's a bit more to it than the four simple rules above.

and then continued on to present my twenty tortuous rules. :)