While I tend to agree with most here regarding the somewhat questionable attitude with which the OP has approached this discussion, I will put in my own two cents.
First off, I would argue that when comparing a light-weight laptop, like the MBA, and an iPad, there will be a lot of overlap in capability, even to the extent that one could argue that each device could do almost everything that the other can. Not precisely everything, mind you, but almost. I would further argue that you could make such comparisons with other types of devices, too. For example: desktop vs. workstation class laptop; workstation class laptop vs. light-weight laptop; light-weight laptop vs. tablet; tablet vs. smart phone. Now, that said, rather than asking the question "what can X do that Y can't?", the better question is, and I grant that this is the question that you seemed to settle on, "what things does X do better and what things does Y do better, and why?".
Now, stepping back into a moment of criticism of the attitude with which the OP has seemed to approach this discussion, when someone offers something that the iPad can do better, if the OP wanted clarification on that, then he should have asked for that, rather than responding with an argument for why that person was wrong and the MBA would work just as well or better in the given situation.
Moving past all of that, which seems to me to be prelude to answering the actual question, I do have an iPad (which I'm using to write this post), and I do have various usage situations which I feel that the iPad excels at far beyond a MBA.
First, there is the reason that I originally got an iPad. I sit on a commission. During our public hearings, we routinely need to have various resources at our disposal. Some of my fellow commissioners show up with several 3" three ring binders that weigh many pounds. Prior to getting my iPad, I tried using my iPod Touch (screen was too small) and my MBP (having a laptop screen up between me and the attendees of the hearings felt like it created an inappropriate barrier). I so disliked both of these solutions that I went back to paper until I got an iPad. Hands down, the iPad is the only effective solution in this case.
Now, when I got the iPad I expected my commission work to be the only place that I would actively use my iPad, but I quickly discovered how wrong I was. While it is true that I have a MBP not a MBA, I did routinely take my MBP with me when I was going places. I quickly discovered that the vast majority of what carried my MBP around for, the iPad was capable of doing quite easily. On top of that, there were a vast array of activities that were suddenly opened up to me. Many, many places have wifi, and because I carried my iPad far more often than I would ever carry a laptop, even if mine were a MBA rather than a MBP, I found myself pulling out my iPad in stores to check reviews of products on site, rather than waiting until I got home.
When I got my iPad 2 a year ago, I got a 3G model, and even more possibilities were opened up. At one point I was in a park with my son and needed to call his grandparents to talk about schedules. I tried their home, their work and their cell numbers. No answer. Now, I didn't really care which of them I got in touch with, and I didn't really want to leave five different voicemails and start a massive game of phone tag, so I pulled out my iPad and sent an email instead.
Before owning an iPad, I never really read an ebook. Since owning an iPad, I vastly prefer reading ebooks. Why? Well, first off, reading a book on a computer screen was truly never comfortable to me. Because of this, I expected to never want to move from real books to ebooks. The iPad changed my mind because of the ability to read in a dark room with far less light than having a lamp on, an important feature when someone is sleeping next to you, as well as the fact that it is easier to always have your current book with you (not to mention much of your library) where ever you are, in the event that you have a moment and want to stop and read for a bit.
Before my iPad, I regularly would watch movies in bed on my MBP. It generally always had to be plugged in, so I was wrestling with the cord. It would warm up, even through a lap-desk. (My wife frequently expressed concerns about certain parts of my body getting overheated, and thus adversely effected by the heat from my MBP). My pad, by contrast, is light, cool, and has a battery that makes it so I usually only have it plugged in to charge overnight when I'm not using it (frequently every other night).
Long and short, there are many, many areas where the iPad excels beyond any laptop, simply from a convenience and ease of use standpoint, many others where it matches a laptop, and many others where it falls short. We live in a world where we don't need to have just one tool, but can have several and choose the right tool for the right job. Many, including myself, have found that the iPad is the right tool for much of our computing needs, with the full computers, be they MBAs, MBP, iMacs, MPs or PCs, reserved for the heavier lifting that they excel at.