A qualitative difference between the iPad and iPhone has been that on the iPad, it is possible to view letter-sized, non-reflowable documents (PDFs, comics and so forth) without zooming and panning. Although the size of the iPad screen, at 9.7 inches diagonal, is smaller than the 14" diagonal of a letter-sized piece of paper, the slightly shrunken images and text are generally readable. Compare this to trying to read a PDF on a smartphone–or even on a 7" tablet.
The larger screen size makes the “10 inch” class of tablets useful as universal e-readers–but it comes at the price of portability. Unlike a Kindle or a smaller tablet such as a Nexus 7, most people need 2 hands to read an iPad. This makes an iPad unsuitable, for instance, for one-handed reading when commuting on a packed subway car. I’ve even found it easier to read print books that are physically larger than an iPad in every dimension one-handed, since you can place your thumb along the gutter (between the pages) and not block any text–but you have to grip an iPad along the bezel.
Incidentally, I have long been annoyed at how little weight the tech press gives the issue of reading non-reflowable letter-sized documents. Detailed reviews of 7" tablets will pass by without a mention of this tradeoff. But of course, most people are not lawyers who regularly must read white papers, law review articles, and briefs that are formatted as PDFs. (PDF is a terrible format for documents that are primarily electronic but that is another matter.)
Apple’s almost-certain-to-be-announced iPad mini (or iPad Air) is likely to have a 7.85" diagonal screen size with a 4:3 aspect ratio (162 mm x 124 mm). A 7.85" diagonal screen gives you 40% more surface area than a 7" diagonal screen. Is this increased surface area enough to make it so that you can view a letter-sized document on a 7.85" iPad without zooming and panning?
I don’t think it is. I set the window size of two PDFs–one Saturday Morning Breakfast Cereal collection (from the recent Humble eBook Bundle), and the leaked Romney/Obama MOU on the conduct of the debates–to close to the exact physical size of the rumored 7.85" iPad’s screen, and while I could read the documents it did not feel comfortable. The screenshot on this post was taken on a 15" Macbook and should be viewed at that size to see what I mean. But notice that the size of the text in the MOU is smaller than the OS X system font (visible in the menu bar). I do not think that text that is this small can be comfortably read at length–it’ll do in a pinch, and might be fine for quick reference. But I have to conclude that a 7.85" iPad is probably not as useful as an e-reader as a 9.7" iPad, if you have to read a lot of non-reflowble documents. (And probably not as useful as an e-ink reader if you just read a lot of proper ebooks, but again, that’s another issue.)
Edit. It occurs to me that in landscape, PDFs and comics may be readable. You would have to scroll to get to the bottom half, but you would not have to zoom. Zooming is more annoying than scrolling.