There's no reason the iPad shouldn't have it's own OS. In fact, it makes more sense than not. The iPad wasn't announced as a giant iPod Touch, it was announced as an entirely new product category:
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It is supposed to fill the gap between smartphone and laptop. So why does it use the exact same OS as a smartphone? Watch the keynote. Jobs says that in order to fill the gap, it must be
better at those things listed above than both a smartphone and laptop.
1.)
Web Browsing - As people browse the internet, they invariably come across media they want to access in varied formats beyond the limited formats supported by the iPad (including flash, but I'll give the iPad a pass). They will browse pages or maps they want to print, and files they want to download. Since the iPad OS doesn't support any of that, how can it be better?
2.)
Email - As people use email, they receive file attachments that they want to save, print, or edit and reattach to another email. It may be music that someone wants to share, an assignment they need to print for class, or a presentation they need to add their slides to and distribute back to the team. Since the iPad OS doesn't support any of that, how can it be better?
Solutions on a per app basis are insufficient. For example, there are some print apps, but they are completely separate from the email app, which is (usually) separate from the document editing app, which is separate from the web browsing app, which is separate from the file storage app. So, you can print from your email in a printing app (after you set up your email account again in the app), but you can't edit it. You'll have to use DocumentsToGo to edit an attachment (after setting up your email a third time), but now you can't print it. It's just a total mess.
As it stands, the only thing the iPad is better at than the smartphone and laptop is using it on a couch. That is, until they inevitably need some basic feature that comes along with web browsing and email, they are going to have to get up and use their laptop, defeating the whole purpose.
As I said, the iPhone OS was a good start, but they really should have added some features like print support, a USB slot, user-accessible file storage with a finder app, and multi-tasking to really create a useful category of product.