I don’t have an iPad (yet), but Tidbits’s Adam Engst nailed it in his latest post:

“So what’s the difference between a Mac and an iPad? It’s that blank slate thing. No matter what you do on a Mac, the keyboard and mouse and window-based operating system make it impossible to ignore the fact that you’re using a Mac, and it’s often equally as impossible to ignore the fact that you’re using a particular program.

In contrast, the iPad becomes the app you’re using. That’s part of the magic. The hardware is so understated - it’s just a screen, really - and because you manipulate objects and interface elements so smoothly and directly on the screen, the fact that you’re using an iPad falls away. You’re using the app, whatever it may be, and while you’re doing so, the iPad is that app. Switch to another app and the iPad becomes that app. If that’s not magic, I don’t know what is.”

You’re not holding a device that runs apps. You’re holding apps that create a different device every time.