Only time and revenues will tell who's right: but I think a few things that emerged from recent discussions are worth thinking about.
It's all about being sexy
What's the ugliest part of an electronic device? Cables. They're messy, and despite every effort to make them look pleasant ...they are ugly, and probably always will. I suspect that the lack of connectors is a way to discourage users from plugging in many devices: famous actresses look a lot more ordinary without the make up. And they hate showing up in public if they have some imperfections on the skin. It's called vanity. The best way to satisfy vanity is to make the iPad an essentially wireless device. Cables are for losers, they're necessary, but only when nobody's watching.
However, if you still feel like you need to upload pictures, connect to external devices... you'll maybe end up buying another device like the AirPort.
Gestures matters
Some folks complained it's less than an iPhone. Wow ...it's big! Do you really would like to phone with a big device like this? It doesn't feel natural. Our body learned to associate gestures with situations, and we phone with small objects (like ...phones) and we read with bigger ones (like magazines, books and newspapers). I think the whole idea is to make the device feel natural. Things or actions that wouldn't fit the device were simply not included.
Looks like the whole idea is to be a device do be used on the couch. Relaxed, and quiet. Like ...reading a book, but not necessarily so.
A pleasant experience
iBook and the relative store, are more than a hint that Apple is engaging Amazon in the book battle. Some friends noted that reading a book requires a different display, to be a pleasant experience for the eye. I think they're right on this. Sill the iPad is not a device to do one thing (as it is for Kindle) but to do many. And not all of them are necessarily known from the beginning.
No multitasking
Some people consider the lack of multitasking capabilities as a key missing feature on the iPad. I think the opposite. Frequent interruptions are stressing, and to me the iPad looks like a thing to do one thing at a time (which by the way is the basic mantra of GTD, Kanban and so on). If you want to torture yourself by being continuously interrupted, a phone or a PC are the tools for you, and this is what you probably use for working. But if you want to please yourself, relaxing on the couch, the only other thing to do while using the iPad is probably having a glass of good wine. Or maybe that's the picture of you that will stick in your brain to make you buy that thing.
I suspect that also the lack of a camera has something to do with that. I mean ...Apple put cameras in MacBooks ages ago: they know how to build devices with cameras. But the stressing feeling of being continuously reachable is what makes people hate cell phones. A device that allows you to surf/read without being interrupted that often, could be loved by some.
It's not for the geeks
Maybe I am a little bit too far on this, but I think the iPad is targeted to a part of the market that had been partially untouched from other Apple products. Think about the agenda: it really feels like a real agenda, with all the potential of an application. There are quite a few users that still prefer planning on paper rather than on a blackberry or an iPhone, cause you don't see the whole picture there. More generally, geeks feel comfortable with complexity and multitasking. Many folks don't.
I have to admit i bought the iPhone for 2 reasons: learning and vanity. The reasons I use it now are completely different, and I discovered them while using it. So I think it's still early to have a precise idea of the potential, and of the marketing success, but really looks like "less is more" has been a design driver this time.