Well, a MacBook lacks touch and cannot detach the keyboard. That is the nice thing about this accessory, that it converts your tablet into a laptop, you still can use it as a tablet though. But you already name the important difference, why a MacBook (and yes, the 12" was a really nice form factor) is better for many tasks: because of the software limitations of the iPad, but they are completely artificial. One could have an iPad as usefull as a Mac if Apple allowed it.

And yes, at least I would hope that if Apple offers an ARM based Mac, it would be close to that MacBook. Though the new Air isn't that much larger.