(1) Interactivity. The “multi-touch” interface is ideal for navigating 2D content. Google Earth is a perfect example. Zooming in, zooming out, rotating or panning are simple and intuitive. Even for less physically dimensional data sets the metaphor of flipping between pages with a flick of the finger or of scaling up and down with two fingers works really, really well. It helps that the system is fast enough that the UI and graphics respond to touch interactively.
(2) Speed. The essentially zero startup and shutdown time make the device seem like a physical appliance rather than a computer. I don’t know how many hours I spend booting up Windows, or on the other end, shutting it down. In any case it’s too long to make it seem like a tool – it’s more like a chore. The iPad, on the other hand – you turn it on and it’s on, you turn it off and it’s off. Unlike a laptop whose state has to be managed, the iPad is more like a book. You open it and use it, close it when done, just like you want to.
(3) Portability. The battery in my laptop pooped out years ago and turned it into a strictly wired computer. Even when the battery works, the keyboard and track pad on the laptop give it a hands-off, indirect feel. It’s like trying to hold a book with oven mitts and leaf through it with tongs, as if the pages were 500 degrees. The iPad, however, is a truly portable device. The form factor, speed and direct UI make this the only computer I’ve tried to take to bed or to the bathroom with me.
(4) Plants versus Zombies. Not to focus on a particular game, but the iPad platform has an affinity for certain types of applications. In particular anything especially two dimensional that requires minimal interaction, like the aforementioned brilliantly designed RTS game. First person shooters will not be flocking to the iPad, but for some kinds of games and interactive experiences it will provide a unique niche.
UPDATE: part 2