[jonathanstrange]

For example, Apple's human interface guidelines mandated that you have to give the user instant feedback (I think they even talked about how many milliseconds of delay are tolerable). A correctly programmed application in OS 9 would give instant feedback on each menu item selected, button pressed, and so on.

They later gave this up and almost everything else in their very reasonable guidelines based on actual research when they switched to OS X in a hurry and multithreaded everything. The early Mail application was a disaster, for example. Generally, people started complaining a lot about the spinning beach ball of death.

In contrast, modern UX guidelines are mostly about web design, how to make web pages look fancy. They also recommend instant feedback, but many libraries and application designs don't really support it.