@I ain't Spartacus
Sorry for saying "mind-share" comment; I've been too long surrounded by corporate wienies.
Anyway, I don't think the USB port is an issue, specially with the digital camera hook-up kit, which basically allows for any USB device to get connected (but, more importantly, the ever important digital camera).
As for not being able to replace their PC, I know some people for whom the iPad has done just that. Oh, they still own the PC, they just tend to not use it anymore.
My parents are looking to purchase iPads for their grandchildren (my sister's kids). My Mother, specifically, saw one at the Apple store and fell in love with it, yet was confused when she heard comments from friends (non-iPad owners all of them) saying pretty much the same thing you did: It's pretty, but it doesn't do anything that your computer doesn't do already, so it's just a waste of money.
She then called me and I asked her to list the things she uses the computer for: browsing the web (for recipes and online tabloids, of course), online banking, and amazon purchases. That's it. (The grandkids play video games on the computer when they visit, too). I told her that those exact things she can do with an iPad, from the couch, easier and better. Plus e-books (which she was considering looking into), and video games for the grandkids, plus--remember that nifty electronic photo frame you got for Mother's day with the pictures of the kids? that too!
That, and her own experience at the Apple store convinced her. She has a Mac Mini at home, so she's already accustomed to using iTunes, so that won't be a problem.
I hear much the same comments from at least one non-techie lady at the office. She purchased an iPad with trepidation after being told by many people "what's the point? you already have a laptop", and discovered what others have: that it is indeed easier and better than doing those things on the PC, and much more fun. She loves it, and talks to all her friends about it.
Make no mistake, this thing is going to take off, big!