Lukas Mathis was cited by Gruber here regarding things that are not obvious in the world of gestures common to the iPad and iPhone.
There was a time when people thought that once we had icons then everything would be obvious and simple. While icons were not always obvious or intuitive at least they provided a graphic representation of an action. Gestures, since there is usually no graphic affordance associated with them are actually invisible and often only discovered by experimentation.
The thoughtful Lukas Mathis article is here.