Some things that make the iPad different:
iWork Suite: not available on iPhone or iPod touch, but AMAZING. In fact, I like it more than the Mac version I have. Its touch interface is amazing, especially on Keynote, but works better than you'd think for all of them. With the keyboard dock, Pages is just as productive as a computer. And in some ways, it all just works better and faster than on my mac, because touching everything seems to be more efficient than clicking everywhere.
iBooks: not on iPhone either (at least yet, but even when it is, iPad still blows it out of the water). Some people will/have bought iPads to use mainly as fancy kindles, and the interface (and massive bookstore) work great
new apps: there are lots of apps only available on iPad because they just wouldn't be productive or worth it to have on iPhone. And there are of course the original 200,000 apps that you can get, many with specific iPad versions. With the much larger screen, you're actually able to use multi-touch in tons of instances (whereas on iPhone, if you've got fat fingers, good luck...)
Music, Safari, Mail, Notes, Photos, etc. ALL BETTER than iPhone's "mini" versions. These new ones are much more intuitive, and have some really cool features. I would still buy the iPad even if there were no 3rd party apps.
As said earlier, the size of the screen really makes everything so much cooler and better. The 3G model doesn't even require an annoying AT&T contract, you just pay month to month. I can't wait until I can afford one...
Hope this helps!