I feel like I've been having this discussion with my clients a lot lately.
(I'm a computer consultant and a lot of my customers have asked me "What I think" about the iPad and "Would I buy one?")
I certainly will be getting one as soon as possible and here are my reasons why it is not just a "big iPod"
Really the only big difference is the screen, but it changes everything and makes it a different device... here are my reasons.
I am a Kindle2 owner and I also use the iPhone when I don't have my Kindle with me. I simply hate reading books on my iPhone (Kindle App) - its annoying to read one paragraph at a time then flipping screen/page. The new iBooks app looks excellent and you can see entire pages, not just a paragraph. (I wonder if the Kindle App will still work on the iPad.)
With the bigger screen we can actually do REAL work on the device. I use iWork a lot on my laptop and it will be nice to be able to open and do minor editing of my iWork & Office 08 docs on the device. It's almost impossible to do that on the iPhone.
Surfing the web will finally be a "good" experience on the iPad (I do not care about flash at all). Yes, I can and do surf on my iPhone, but it is also annoying to be constantly scrolling and zooming just to read a simple news article... or even these forums
I do not watch videos (much less a digitally downloaded movie) on the iPhone because I can barely see anything on the little screen. (This could be me as I am going to be 40 this year and will be "officially" old by my daughter's standards.) The iPad screen is bigger than most portable DVD players so it should be a much better portable movie player. I might actually download a movie or two now.
At least from the demo video, it looks like that larger screen will allow for a much better typing experience on the screen as well...
So just that larger screen makes the iPad a much better device (for certain things) as well as a different device and all around user experience than an iPod or Touch.
Also as someone else said earlier, that bigger screen is going to allow for some amazing apps that nobody has done or even thought of because they were so limited with the Touch or IPhone.
Just my opinion..
And I still can't wait to own one... where's that damn pre-order email?