There are just a few things missing from iOS that would make it feel like full-scale OS X. These things could be added transparently, either in the baseline system or as an add-on Package, in such a way that grandma could continue to use her iPad just as-is without ever having to be aware of them, but her grandson could access a richer user experience.
• Finder
Many users are just fine without direct file system access, but for some, it is not a computer without it; this would probably also include Terminal.
• Windowing
Several times I have wanted to window two or three apps. Making a windowing paradigm work on a touch-based UI is a bit of a challenge, but iOS 5's four-finger touch scheme provides a nice alternative to drag-bars, close/minimize/zoom buttons and resize widgets. Implementing drag-and-drop is another challenge that would come along with this.
• Menus
It seems obvious to me that straightforward, unmodified menubar addition is one reason MS had trouble gaining touch traction. Menus have to be presented differently and, in the case of a touch interface, called up differently. The most sensible first approach would be to establish a standard for CMs, such as, for instance, touch a thing, then tap a second finger to pull up the CM. I could see the static menus (menubar) hiding under the status bar until you give it a lateral swipe, then appearing in a submenu cascade that you would navigate. Yet, somehow, menus have just about run their course. For convenience and flexibility, some new command scheme that lends itself well to touch (as well as mouse) needs to be devised.
• Targeting
With iOS, we still have a problem in some apps with these sausages we use in place of a cursor, especially graphics and web browsing with tiny links. Until we either have a clear and simple way to watch our finger position or easily use a mouse and cursor, serious apps like Photoshop will remain just out of reach.
iOS is inching closer to a serious desktop alternative, but a few obstacles remain in the way of that. One of the biggest is Apple: they love their closed ecosystem and the revenue it generates for them, allowing users to get into the device and use its full capabilities would compromise their bottom line. For this reason, having a Mac-like iPad will not be a free upgrade.