I disagree with this. The iPad is a computer, with a different operating system.
The iPad is a restricted, closed platform and by that its purposes are limited whereas a computer by the very definition in our common understanding is a true multi-platform device without artificial restrictions. iOS does not even give the user a common file system that can be shared by all applications, nor does this operating system allow for any customization.
What can't you do on an iPad that a Tablet PC can do?
On a tablet PC, I can install and use any programming language that I want and write software with it - and that's just for starters. Apple recently didn't even approve a teaching programming language for kids in the AppStore because they deemed it a violation of their EULA. Imagine that: Children would be able to write software for the iPad without having to use Xcode and Objective-C on a Mac, and they would have been able to pass on their little programs to other kids without having to go through the AppStore's approval process! What were they thinking! No, Apple could never allow children to do that!
If it wasn't so sad, I'd say that this was by far the most ridiculous decision Apple has made in the last three decades.
Besides that, a Windows based Tablet PC, for example, can run real world software like Microsoft Office, whereas the iPad only has a stripped down version of iWork. Have you looked at Pages for iPad? It has basically the feature set of Microsoft's WordPad, which is a freebie in Windows.
Android tablets at least have an Open Source operating system on them that can be customized to Death if you want to. Nobody is restricting your choice of programming languages, because nobody could - it's Open Source, remember?
The usage for a Tablet is for taking notes, email, web.
Haven't you just said that it is a computer?
Not game, design websites and edit movies. That's what the iMac and MacPro is for 😉
Well, I guess that by your definition successful multi-touch games like Plants vs Zombies HD don't have a place on the iPad then, and neither has Shredder Chess and all the other board games that have been "ported" to the iPad.
I think you're dead wrong. Besides being the couch potato Internet designer surf board of choice and an eMagazine (not eBook) reader, the iPad's main purpose will be that of a mobile game console. Why? Because it actually royally sucks as a writing device, which rules out heavy email writing and note taking.
But I'm sure that you will soon see iMovie and Garageband on the iPad. They'll be rather simple compared to their Mac OS X siblings, just as the iWork apps for the iPad are complete toys compared to their Mac siblings, but you will be able to do something with them and they'll be easy to use. And, as always, for most regular people it'll be enough.