Honestly I think primary reason that iOS tablets have been successful is because they have a different OS. Fundamentally iOS is far less capable but also far less difficult to use and fragile than a full-fledged OS. A tablet is less PC and more consumer electronics. And that's not a hardware distinction, it's a software distinction.
It has a great deal to do with UI.
When you learn to write an app for iOS or Android, the first thing you learn is that your app needs to be ready to freeze and save state at a moments notice, and then re-hydrate itself later, also at a moments notice. This is the fundamental difference in the OS, not the touchscreen and widgets.
This is seen in OS X when an app window appears greyed out with a progress spinner in the center. The app may start out in this state if the machine was hibernated, or it may go into this state if it is open but it fits certain criteria, like all its windows being occluded by others. When this happens, the process is killed.
So then the question is - why have a separate OS for tablets as compared to laptops? The current reason is not so much to do with the UI but more to do with management of limited resources. OS's like iOS and Android are very aggressive about preserving memory and processor cycles as a way of squeezing extra life out of batteries.
On phones that different-OS resource advantage will probably continue. But on tablets, its less clear. As the intel chips get better at low power, a tablet running a 'full fat' operating system becomes more feasible.
This is what the Surface Pro 3 shows, whatever you think of it as a device: You _can_ run a full OS on a powerful processor in a fairly thin, fairly light tablet. Not the thinnest, and not the lightest, but still it raises the question: Why do tablets need a different OS?
Microsoft are tackling that question now, I think Apple will ultimately need to tackle it as well.