Apple's game-changer
Indeed, because Apple designed a touch-screen, gesture based UI for the iPhone, and didn't try to squeeze a desktop OS designed for large screens with keyboard & mouse input into a mini device without any of these things.
Like you, I am a big believer in the importance of tailoring the interface to the nature (size, intended use, capabilities, etc.) of the device. Clearly, both the iPhone OS and OSX for desktop/laptops are very wrong for a 'tablet' device. If the iPhone OS is used, it's just a big iPhone and not a thing in itself. Likewise, if OSX is used, it's just a stripped-down laptop.
Instead, I believe what Apple is pursuing is a new thing entirely. What is it? Well, according to accounts of Apple talking with publishers, I believe Apple wants to be in the publishing business in a manner similar to the way the iPod put Apple into the music business. That is: As the world's largest distributor of the product (in this case, newspapers, textbooks, magazines, paperbacks, etc.).
This would be a product of stunning impact if done right. It would be the modern-day printing press. The internet itself is just that now, of course, but the 'net also distributed music at one time. You'll recall, though, that during the Napster days, music publishers made little or no livelihood through music distribution over the 'net, and tried to shut it down.
Today, it's likewise extremely difficult to make internet publishing profitable. I believe Jobs sees this as an opportunity that will make Apple the center of much of the publishing world. Publishers fear loss of their own power, and with good reason, but like the music publishers and distributors before them, they lack the tools to fight back. (And that's not going to change.)
The tablet (whatever it's called) is a game-changer. But it must be a 'thing unto itself' to accomplish this. It will not be a large iPhone or a small laptop, but it will appropriate techniques perfected in those devices and apply them to its own unique ends.
Apple has I believe been studying how users interact with their newspapers and other printed materials, in order to give their new product a unique flavor, just as the iPhone has a unique flavor. But there are other uses for a 'tablet' form factor, such as watching videos, taking notes, playing games, and drawing. Apple will not neglect these, since they represent more reasons to buy the thing. Equally important, they represent more reasons to carry the thing around.
The laptop = Primarily: moblie workspace, web browsing, email. Secondary uses (for the majority) are to watch web video, play games, hear music.
The desktop = Primarily: stationary workspace, web browsing, email. Secondary uses (for the majority) are to watch web video, play games, hear music.
The iPhone = Primarily: verbal communication (it's a PHONE), playing music, viewing personal photos, running highly task-specific apps (mainly for gathering info), short text communications. Secondary: email, net browsing. iPhone apps differ greatly from work apps - this is primarily a 'leisure' or consumer device. Not terribly well suited for watching videos or general web browsing, though it is handy and will do in a pinch.
The tablet = Primarily: Your daily paper, your favorite magazines, textbooks, novels. The Kindle but with many more uses, more power, and color. Net access to your reading material of choice is FREE with a subscription to that publication. (Some ad-supported publications may be ENTIRELY free.) Important secondary uses: Games, video, email. The bigger form factor makes it more suitable for these things than an iPhone, yet it is more simple and portable than a laptop (appeal similar to a netbook).
What about Photoshop, etc., on this thing? I think you wind up with "Photoshop-lite" versions for those who want it. The main reason to buy the thing is not to run Photoshop, but I think Apple will encourage stripped-down versions of popular apps.
What about voice? I doubt that Apple wants to deal with the phone companies any more than it already does. But of course, the thing will have cell and WiFi capabilities, along with a camera and mike. You won't be able to use it as your primary cell phone, but you can still make Skype calls. The cell will deliver your reading material from anywhere, and may also enable 'net browsing from sites willing to foot the bill.