[shuttlebrad]

Depends heavily on what you're letting people do, and how long they're going to spend doing it. Games tend to have smaller numbers of "verbs", and longer engagement times - for a superb example of progressive teaching, play through the Portal developer explanation (available after you finish the game).

It's much, much harder to do this with an application where you can't boil the functionality down to a small set of verbs - line of business apps are the usual terrible example of this.