It's been said on innumerable tech and Apple blog sites:
There is no tablet market, only an iPad market.
I think that watching the recent withdrawal of HP from personal computing, and the rumours that Samsung have sold only a tiny fraction of the million units shipped of their Galaxy Tab it's easy to find the truth in this statement. If it were simply a matter of hardware or form factor then as with the smartphone market Apple would be just one contender of many, probably still the largest single manufacturer from a hardware perspective and second in software penetration as it currently stands between iOS and Android phones. It's a clear and predictable pattern.
This hasn't happened however.
I think it's really a matter of Apple reaping the rewards of their walled garden approach to content curation. The strength of the iPad resides in iTunes, iBooks and the App Store and the seamless delivery of content that particular ecosystem provides. It's this easy, uncomplicated approach to managing your media library that makes the iPad more appealing than just a comfortable, portable touchscreen which anyone can produce.
I think therefore that the real challenges to the iPad are just lumbering onto the horizon now. Two companies that have the existing technologies and weight in the content marketplace to provide their own integraded media ecosystems are Amazon and Sony. Both are about to muscle in on the iPad's turf. Remember that it's widely believed that Amazon are providing much of the infrastructure for Apple's forthcoming iCloud and has its own music and app store already in place.The expertise and infrastructure is clearly ready to go. These two probably have the best chance yet of offering users the same kind of experience that the iPad offers.