An 11" Mac Air has a footprint not all that much larger than the iPad (7.5" x 11'8" vs 6.5 x 9.5") and, while over twice as 'heavy' (2.38 lbs vs 1 lb) it is a real computer capable of running full (and familiar, even to PC users) software and connecting to real peripherals - like external drives, printers etc.
The iPad, however good, has never been intended by Apple to be anything more than a companion device to a real computer - and while there are plenty of apps out there to squeeze the most out of the product, Apple has been very careful not to let the device cannibalize users from its fully-featured products.
That said, the Mac Air has an operating system that will require a learning curve from PC users (not nearly as 'intuitive' as the Macoyltes insist it is - there's a reason there's a healthy market in 'Switching From...' manuals out there), while the iPad has an OS easy (or basic) enough that even a toddler can use it. And yes, you can add hardware keyboard to the mix on an iPad, but then the weight goes up, so what are you gaining?