When the Mac was originally designed, they came up with this cool feature whereby menu operations would leave the menu title highlighted until the command had actually completed. An example is shown below.
This is great feedback for two reasons. Firstly, it informs you (subtly) that the current task is still in progress and that it's pointless to try any more operations until it completes. Secondly, it confirms that your menu operation is taking effect, particularly useful if you were doing that operation with a command key.
But all of this has been in the Macintosh human interface since Day 1, so there's hardly anything special about it, certainly not enough to justify a human interface subtlety entry.
Unfortunately time has caught up with the Macintosh and the effect simply does not work as well today as it did in the past. Why? Because our machines are now going so fast (a Power Macintosh G3/300 is hundreds of times faster than a Mac 128) that simple operations (such as copying some text) happen so quickly that you don't get chance to see the menu highlight. For a good example of this, edit a document in SimpleText and then hit command-C. On any 68040 machine or faster, you simply cannot see the menu action take place. I often find myself typing command-C multiple times just to make sure.
I was talking this over with Peter N Lewis one day and suggested a solution, which was to put a lower time limit on menu commands, such that all commands took at least a tenth of a second (6 ticks) to complete. The implementation of this is trivial, Peter put it into Anarchie the next day, but the effect is astounding. Try hitting command-C in Anarchie. Contrast its effective feedback with the poor feedback given by SimpleText.