If you remember, Bill Gates has bet on handwriting and speech recognition and text-to-speech. Wrong choice, the real next-gen devices seem to be (are?) multi-touch and accelerometer-based devices (as the iPhone and the Wiimote).
Related: What's the difference between an accelerometer and a gyro(scope) ? While it seems to be pretty complicated (angular momentum?), a basic difference is that acceleromer-based devices cannot sense their rotation with in a place perpendicular to their "ground" axis, that is, the axis that points to the ground.