The Killer Feature?
The combination of Snow Leopard and your fingers! It's as simple as that. All those strange cover flow in iTunes, in the finder, everywhere just does not make much sense with the keyboard and the mouse, but it just fits for your fingers (see the iPhone).
Imagine putting all your holiday pictures on the new device (sd-card slot!), using your finger to browse them, alter them with iPhoto or Picasa, sort them and show them with music to your familiy and friends. Not at home, but after dinner at the hotel pool. Or put your video footage on it and use your fingers to edit a small movie! The iPhone is great to use on your own or to show stuff to one or two friends. But it's just too small for a larger group.
Or how about playing all the board games of the world on it with your whole familiy like an animated chess or monopoly? Watching movies of course? Drawing pictures?
It's gonne be the portable computer for all the computer haters. And with the bluetooth-keyboard you can even use it to do word processing.
It'll habe iLife on it und you can put iWork on it. Read and edit pdf, word documents, presentations while on the go. And yes: Download new software from the app store und start using it as an ebook-reader especially for newspapers and magazines.
It's gonna be like the iPhone: Not the first device of its kind in the world, but the first really useful and innovative one for a very long time.