When I grabbed the iPad Air, I wanted to juggle it. So I did. I threw it up in the air a bunch of times, like you do with a giggling baby. I did so gingerly, like you do with a giggling baby, but it's the first 10-inch tablet that feels effortless.
The Air's weight is its killer app. You can't see this. You have to feel it. At just 1 pound, it comes very close to the tablet ideal of a magical sheet of paper: thin, durable, easy to hold in one hand or two for as long as you want.
I have a 4th-gen iPad here, and compared to the Air, it's a leaden thing. It's not the thickness, it's the weight; it weighs down your wrist in ways the Air doesn't. The iPad has never felt to me like something you could just whip out. It had a bit of deliberateness to it. The Air removes that boundary.
It's so light, I'm afraid it makes the iPad mini irrelevant. I held it with one hand for a while, and my hand didn't get tired. (The mini is 12 ounces; the full-sized Air is 16.) And it's not like the mini's chubby form leads itself easily to one-handed use anyway.