One of his points is that reusing old metaphors is a terrible way to design a computer program. It's freeing to catch Cooper's vision about breaking with the past and using all sorts of cute new widgets instead of the tired old ones built into Windows. One gets the feeling that using a subtlely different gizmo for each new purpose would result in a much more powerful interface. Another interesting proposition is that error boxes should never be presented to the user. To the user, being forced to say OK to an error message is annoying and humiliating.
Cooper instead suggests that the program deal with every error in an intelligent way.
It can subtlely show inaccessibility of some component, or can fix the syntax on a user entry, or whatever. Of course, it would take a lot more effort on the part of the programmer to implement such error handling, but I agree with Cooper that such a system would be far more fun for the user.