+1 what cuhnool said, but to which I will add:
1. Certainly the iPhone/iPod Touch and the iPad have a lot in common, starting with similar-looking hardware. When its switched off, and perhaps to some even when its first turned on, the iPad just looks like a big iPod Touch. I grant you that.
2. The thing is -- it won't be USED as "just a big iPod Touch." Because of its larger size, it brings an
entirely new dynamic to the multi-touch screen. On an iPod Touch, your finger takes up (and effectively blocks) about a third of the screen, meaning that what you are able to do with it is limited. On an iPad, the bigger screen makes the influence of your finger at least 4x more accurate, which opens up a lot of possibilities
in software that just cannot be done on an Touch. Gestures that would be awkward on the iPod Touch can be accomplished on the iPad. iWork on the iPad is just one example -- that same app would be utterly
impossible to use on the iPod Touch. Likewise the revised Mail, Contacts and Calendar. It's not JUST more screen space, its the freedom for developers to take advantage of what the iPad brings to the table.
3. Likewise, the new processor means, at least for a while, that a lot of functions and power in the iPad can be leveraged in ways you cannot manage on an iPhone. We've only seen a bare hint of that power in the demos so far, but I did notice for example that Google Maps and video player were FAR more responsive than I've seen on my iPhone, suggesting that Apple has added dedicated onboard components for specific jobs (like decoding H.264) that you can't fit in a Touch. This too will be documented to developers and exploited, resulting in apps that could conceivably
require the iPad's architecture. Think about how the iPhone went for seven months without any apps other than the ones Apple shipped, and the App Store was not an overnight sensation. Think Apple won't pull that rabbit out of the hat again for the iPad?
4. You may not have noticed it, but Apple barely paid any attention to the iPad's built-in iPod. Why was that, if the iPad is just a big iPod? The reason is because although it CAN play music, that will almost certainly be the LEAST used of its main functions (not counting playing music in the background while other apps are running). This is Apple's first totally new product in a VERY long time that didn't have music as its main focus. This is because the potential of the iPad far outweighs the (now realised) potential of the iPod Touch.
5. Though we have only seen a demo of it, it's already pretty clear that reading a book on the iPad will be a hugely different experience (not just "bigger") than it would be using (say) Classics on the iPod Touch. They didn't go into it, but iBooks is likely to have features normally found on the Kindle rather than just the same feature set of something like Classics (which, incidentally, I love) running pixel-doubled.
6. The social aspect will be sooo different with this product. Let's say you were surfing on your iPod Touch, and found a funny LOLcat picture you wanted to show your wife, sitting on the other end of the dining room table. Or a great YouTube video you wanted to show everyone in your living room. With an iPod Touch, you have to either hand the device over and let it pass round the room, or everyone has to be within a foot of you. It's kind of awkward sharing things with someone else, even those very close to you. With an iPad, you just flip it over, and everyone in the room can see it (and probably hear it if they're quiet). Or anyone sitting next to you can enjoy what you're watching exactly as well as you can. The iPad isn't just bigger, it has an entirely different and much higher-quality screen on it than the iPod Touch does. Don't underestimate how this factor will change how people interact with it, or the impression it will give others in terms of a desire to buy.
7. Built in mic means that Skype and other such voice apps don't require a headset mic to work (though of course you have that option). Very nice.
8. Real bluetooth that supports lots of different kinds of devices. Can't get that from a Touch either.
9. Connecting it up to a projector. Ever since the iPhone came out, I've been wanting to hook it up to a projector and show it a) running live and b) running a slideshow. I can use the iPhone as a REMOTE, but I still needed a laptop to create and run the actual slideshow. Not anymore I don't.
10. It may not fit in a pocket, but it DOES fit rather perfectly in a folder, a notebook, a knapsack, a purse, a briefcase -- even Stephen Colbert's tux!
Its just so much more portable than a laptop, and (perhaps this is a big YMMV thing, but) I don't tend to do much "computer" work when I'm away from my workstation, I tend to want to do more "relaxing/recreational" things; surf, read a book, check my email, play a game. That's what I tend to do when I'm in a coffeeshop, and that's why I don't bring my laptop to coffeeshops much anymore. But even when I *do* have my laptop with me, I still *tend* to do that "light" stuff (maybe work on a slideshow or do some writing at the most!) when I'm using the laptop in waiting rooms, airports, airplanes, ferries, etc. The iPad truly DOES mean I can leave the laptop behind almost 100% of the time, and perhaps even trade it in towards a bigger and more powerful desktop for the home.
11. Seniors and other non-nerds would LOVE a zero-configuration, zero-maintenance computer that's actually large enough for them to see. You kids of today with your sharp eyesight (grumble grumble) can do most of that on an iPod Touch, but people of a certain age need something a) more accurate and b) larger than that for us to use it the way a teen might use the iPod Touch. Not just
bigger, mind you (we're old, not blind!), but bigger AND
more accurate. I promise you this much -- middle-aged and senior non-nerds are going to
eat this thing up, and this is the exact same group that is generally pretty "meh" to the iPhone and iPod Touch.
12. Lastly (at least for now till I can get my hands on one), that dramatically longer battery life will make a huge difference in how its used. At least IME, the iPhone and iPod Touch crap out pretty quick with sustained use. The iPad's claims haven't yet been really tested, but Jobs indicated that with "normal" use (let's call that "using the apps it ships with" for arguments' sake), it should last all day. This means the iPad represents more freedom than an iPhone or iPod Touch can generally give me, allowing me to further divide my "real computer" tasks from my "lighter tasks." I could see a lot of people basically leaving the iPad on the coffee table or their nightstand almost all the time, using it as the "den computer" or the "bed computer" so I can spend less time at my "desk" and yet don't have to lug so much "equipment" around to achieve "mobility."
So maybe I should have said "it's not JUST a bigger iPod Touch" and "not JUST an iPod on steroids," but hopefully you get my drift.
When the iPhone came out -- when the iPod first came out -- its spec sheet really didn't make it stand out from its competitors at the time. In the case of the iPod, it really took about three years and three generations before it dominated the industry. The iPhone did it quicker, but still did have that seven-month period where it was very unclear if Apple would even meet its first-year sales goal.
But look at how fundamentally both products changed -- and the success they enjoyed -- over time, even without huge changes in hardware (the iPhone has essentially the same feature set today that it did when it came out 2.5 years ago!). I think the iPad is a continuation of Apple's recent trend in making hardware that is in itself not full-featured (the aluminum keyboard, the magic mouse, the iPhone etc) but really shines once you add in the software. I have no reason to think the iPad will not stand apart from its iPod Touch "cousin" more fully as the rest of 2010 goes by.