I've been having this discussion with friends all day. Here are the problems with the iPad as I see it.
1) It runs a modified iPhone OS. That means (as of now), you can't run multiple concurrent apps, you can't install your own software if it isn't on the app store, you're severely limited in terms of customization and in terms of performance modification. You're locked out of almost EVERY important system. This also means you won't be able to just hook up a camera and use it as an external drive or controller.
2) You're locked out of the hardware. It has non-removable 64GB storage (at the MAXIMUM), a non-removable battery, and no way to access outside media (i.e. CD/DVD). You will theoretically (eventually) get access to remote storage via USB/WiFi, but that won't be directly Apple supported and will be through an app or hack of some kind.
3) The 140,000 apps currently on the app store are MOSTLY useless. I own an iPhone and can count on one hand the number of apps I actively use. Sure, I've got 30+ currently installed, but most get used very rarely.
4) Typing on a touch screen is OK for a phone, but would be terrible for word processing. That basically rules it out as any kind of laptop replacement (if you're considering that, which even Apple tells you it isn't.) Its slow, you have to look at your hands because you have no tactile difference between where keys end and where keys begin. So try typing up a long email or a paper/report on a touchscreen, and you'll end up spending far longer than you should.
5) Touchscreen gaming isn't that good most of the time. Ya, they're trying to reach out to gamers, but honestly, as a huge gamer myself, I don't want your app store games on a $500+ piece of hardware. I'd better be able to play my retail games, when I want them. Also, has anybody else tried a first person shooter on their iphone/itouch? I have, it isn't good. Without an actual button/stick to control where you're aiming it can be very difficult, let alone accidentally repositioning a finger off of where your "fire" button used to be and realizing that you can't shoot the bad guy running straight at you. There are only a few types of games that "work" on touchscreen right now: puzzles, cards, sudoku-type games, and top down strategy (such as tower defense). Maybe a decent racer here and there. Outside that, touchscreens are a long shot from a console, hand held, or PC for gaming.
6) eBook readers are BETTER for eBooks. Why? eInk, plain and simple. Reading on an LCD, even the best LCD's, is tiresome. Your eyes get tired very quickly. I don't want to sit at my desk and read a novel on my monitor. Its hard enough to read through a long blog post without the eyes getting a bit strained. This is not the Kindle killer (the Nook wasn't even the Kindle killer for that matter.) If you want to do eBooks, you're better off with a Kindle (or Nook, or whatever your fancy is, but probably the Kindle).
7) You don't have enough control with capacitive touchscreens at this point. They showed off the "Brushes" app, and it did look pretty good. But since you can't use a stylus, your finger is what you've got. And there's a reason fine artists (and computer artists) use a brush/pen/pencil or Wacom tablets rather than their mice or their finger - its much much much MUCH more controllable and much much much MUCH more precise. Plus touchscreens aren't 100% accurate in determining WHERE you put down your finger. Its usually pretty close, but pretty close isn't always good enough. And honestly, unless you have TINY fingers, your finger takes up a fairly large amount of screen real estate, so your touch area is fairly large as well. Meaning trying to do precise or intricate actions is very difficult.
OK, so that's all the bad stuff. Some good stuff?
1) That NYT app looks good. I must admit I liked seeing the newspaper LOOK like a newspaper. The embedded video and popout photos were pure gravy.
2) Watching video will be great. I like the idea of having a 64gb video player to take around with me, that I can also check my email/do web browsing on. But not for $700+ thanks. EDIT: The bad part here is its a 4:3 display...not even widescreen? Whats up with that? Who uses 4:3 for ANYTHING anymore??? I'm moving video to an inbetween category...
3) Better iTunes functionality is key if you're talking about using it as a replacement (or sometimes replacement) laptop. Having good control over media is a nice touch (though not really necessary).
4) A cheap 3G plan (with "unlimited data") means that with some hacking you'll probably see tethering capability. And you'll be less likely to get in trouble for it than tethering your iPhone (or other smartphone without tethering plan).
5) The form factor. This is personally the BEST thing about the iPad for me. I love the size and weight, and how thin it is. I really hope we see some Windows tablets replicate this, because with this form factor and Windows7, I could see myself actually going for a tablet.
As some others have said, all this is, really, is a large iPod Touch with the possibility of running iWork. There is pretty much nothing this can do that the iTouch/iPhone can't.