Consider that my iPad has Inkling textbooks, both as references and class books - in full glorious chapters and content.
Zinio lets me subscribe to Magazines for next to nothing in full glory.
Netflix can stream for 10 hours seamlessly and integrates perfectly.
iBooks, Kindle, Nook apps etc cover literally every book you can desire to purchase.
GarageBand lets me take with me a Grand Piano and numerous other instruments for on the go accompaniment
Office is a snap, with the plethora of document editors that integrate with DropBox etc
Flipboard/Feedly etc puts relevant in the ease of use
Atomic gives you full website surfing, adblocking, tabbing, awesomeness while Skyfire gives you Flash - even when most of use end using ClicktoFlash and other extensions to DISABLE FLash on our Laptops to save battery life.
Need I mention the hundreds of other dedicated and generic uses for the iPad (sharing, social networking, work, media output, streaming, skyping/facetiming, even drawing and painting etc etc etc etc etc)? No, it would take too long, and Android users will run out of Notification LEDs to put to as their ultimate productivity advantage.
It's not just the iPad's smoothness, incredible screen, battery life, built in 3G and GPS capabilities... it's what they facilitate, which is productivity. Much like a computer or operating system is useless without the applications it can run and the calculations it can do.