> and it’s taken them this long to learn how to quit an app, as opposed to simply closing its windows.
Well that's the whole point: they shouldn't need to know the difference. The system should take care of that and beginning with Lion it will.
> As another example, let’s say someone who’s never used Expose or Spaces accidentally activates Lion’s new full-screen feature in Safari. Apps that go full-screen create a new Space to live in, and, obviously, they fill the entire screen. I don’t know about your parents, but the phrase “how do I get out of this” rings in my head just thinking about it. So does the needlessly lengthy phone call that would follow.
So you tell them "swipe left or right with three fingers" and then they'll know. Software doesn't need to be immediately and completely obvious to everyone in every situation. What's important is that basic concepts are universally applicable once they're learned.
And keep in mind that discoverability does not mean every possible action needs to be visible at first sight. Swiping should become immediately clear once you fool around with the trackpad; the feedback is obvious, immediate and pleasant.
> Use another app, and the same gesture could mean something completely different.
I don't think the slight differences in the minutiae of swiping gestures (one finger, two fingers, three fingers,...) is that big of a deal. There is (or should be!) immediate feedback and the actions are always (or should always be!) nondestructive. It should not take long for anyone to figure things out.
Granted, there is always the problem of especially older people being less... "playful" in how they approach new technology. Most often that stems from a fear of breaking things. Which is exactly why it's so great that we're finally on the right track with the latest trends towards "care-free" computing environments: autosave, unlimited undo, auto resume, automatic backup and synchronization etc.