"A simple example of good design is the 3 1/2-inch magnetic diskette for computers, a small circle of 'floppy' magnetic material encased in hard plastic. Earlier types of floppy disks did not have this plastic case, which protects the magnetic material from abuse and damage. A sliding metal cover protects the delicate magnetic surface when the diskette is not in use and automatically opens when the diskette is inserted into the computer. The diskette has a square shape: there are apparently eight possible ways to insert it into the machine, only one of which is correct. What happens if I do it wrong? I try inserting the disk sideways. Ah, the designer thought of that. A little study shows that the case really isn't square: it's rectangular, so you can't insert a longer side. I try backwards. The diskette goes in only part of the way. Small protrusions, indentations, and cutouts prevent the diskette from being inserted backward or upside down: of the eight ways one might try to insert the diskette, only one is correct, and only that one will fit. an excellent design."
I thought that this passage was interesting because it describes the excellent design of the floppy disk, a very simple item. The passage describes that only two sides can fit, and that of those two sides, the indentations and grooves along the surface of the floppy disk only allow it to go in one way. It shows that even when designs are simple and may seem confusing, after simple trial and error some objects can easily be figured out as long as they have the right constraints and affordances.
I think that Norman's book continues to be influential to designers even today because it is very well written and the concepts of the book are very concrete. Many of the points made in his book are still valid today, and many of the ideas that he presents will always hold some relevance to the world of design.
If I were to evaluate the design of a product based on this chapter my checklist would include visibility, constraints, simplicity, ease of use, affordances, and mapping (preferably natural mapping).