There's my generation, the digital generation -- we were born into an analog age, and lived through the digital revolution. But we'd better be ready to move over for the touch generation, born into the digital age, but growing and learning on the brink of the touch revolution. The iPad was the shot heard round the world, the beginning of the end for the digital revolution.
The Macbook Air is sort of a final salute to us, the digital generation, the people who still feel more comfortable with keyboards and multi-windowed interfaces. We're on the way out, but Apple is making our "later years" more comfortable.
The huge difference rests in the market applicability: Digital-age computers will always be useful and intuitive to the minority of the world population, let's say the 20%. Touch-generation appliances are intuitive to the other 80%. The future market is in Touch. OSX Tiger is a bid to allow Digital-agers the benefits of the Touch revolution, but it won't bring the Touch people into the Digital OSX camp. Yes, there will (almost) always be high-powered computers with physical keyboards (for another half a generation or so anyways) for specific purposes, like developing applications, but still they're going to go away, just like punch-card computers are no more. Touch is just more versatile, more intuitive, smoother, and less of a buffer between mind and machine.
In short, the Air will never have the mass-applicability of the iPad, yet as far as the current computing market goes (don't forget it's the 20%) it's a huge windfall. Instant-on, oh so light, and oh so quiet, it's halfway there, but only halfway. Don't get me wrong, multitouch on my Macbook Pro is addictive but only compared to the lack thereof on the PC -- it's nothing close to the potential of what will be with the iPad. Our children will never understand how we used to interact with our devices via keyboard and mouse.
Maybe it'll look like this: