Ben Bajarin gives a brilliant account of the advantage that the iPad has over other PCs, in “The iPad Advantage” ($). In particular, this paragraph absolutely hits the nail on the head:
The PC is for certain a general purpose computer. Yet its form factor limits all its general computing capabilities to only be taken advantage while in a fixed position either at a desk, or with the device sitting on your lap. The iPad, and the slate form factor take this idea of mobile general purpose computing to an entirely new level. The iPad enables its general purpose computing power to be used in both stationary and mobile situations. The iPad liberates general purpose computing from the lap or desk and enables it in contexts where computing was absent before.
The iPad is usable pretty much everywhere, and that on its own increases its power compared to other PC types. I’ve used my iPad to write hundreds of words on the London Underground, something I’d never do with a laptop (mostly for fear of impaling people either side with my elbows).