The tablet PC uses mouse-style point-and-click (with a stylus) navigation. The iPad uses the iPhone's finger navigation, where windows are easily opened, closed, and resized and content is browsed with intuitive finger motions.
The iPad is a whole new ballgame. Competitors' trash-talk is going to hit Keith Olbermann-like hysteria levels over the next week, but it looks Apple has lapped the field with this device.
I have a friend who owns a tablet PC running Windows 7 sitting next to me at this moment and he is using his finger to navigate. You don't necessarily need a mouse or stylus - although you do have the option if you wish.