If you're building a tablet from a phone OS, you would fail to have a completely stand-alone device, in the sense that a laptop is completely standalone. You couldn't have file access to dump photos, video and other media onto, you'd have to sync it to something else once in a while to get everything you need. And you have to go through a marketplace instead of installing stuff like a computer.
There is also no real way for apps to interact with each other. There's copy and paste on smartphones, and certain apps can read data files from certain other apps (like the contact list), but there's no way to interact like dragging and dropping files across applications. In the iPhone, you can't even multitask to work on two things simultaneously. You can on Android, but there's minimal interaction between applications. That's not saying it can't be done, it's just not so entrenched in the base OS or the base philosophy that application developers don't do it very often. If the OS maker doesn't do it, developers won't either.
Also, because phones are a very isolated experience, App Stores make it much easier to find apps that are both customized for your device and safe to install. This is great for phones, since stability is important, but when you're getting into higher-performance devices, you want the ability to choose what apps you want, not just pick from the ones that Apple or Google deem OK for you to consume. And since this kind of tablet is adapted from the phone ecosystem, that's the only choice you have.
To have a very good experience on any sort of serious computing device (not a phone), you need interactivity. An example on the Mac is the way your Mail application knows if someone is online in iChat, and shows a little light by his name, telling you that you can just IM him instead of emailing.
Peripherals is something else a phone-based OS can't handle well. You're limited to a specific number of device accessories that needs to be vetted in order to ensure compatibility. Even the iPad, which has a few more accessories than the iPhone (like a keyboard), doesn't have nearly the amount of compatibility as a desktop. A tablet needs to learn this lesson from desktops in order to be truly useful. Plug in a keyboard? Sure. A firewire camera to have the device act as a target storage device? Absolutely. Another tablet, so you can have twice the amount of display area? Why the hell not. Print? Yes.
But not all of this is software. As the size of a increases, your expectation for power does too, and battery life decreases in accordance. So theoretically, in a tablet device, you'd want to have one significant step up in performance over phones, which we're not seeing in these devices. I'm not talking just running the same applications faster, with upscaled graphics, I'm talking entirely new things you can only do with increased processing power. Stuff like true multitasking, games that are actually noticeably better than cellphone games, light media editing (not as good as a laptop, of course) and media playback of all kinds, handling all sorts of codecs.
That's right, people expect more functionality and power with that bigger screen.
