The underlying problems, things like the lack of multitasking, expandability, the anemic iBookstore selection--all that stuff has been covered in the initial reviews . It's something else entirely to actually have an Apple  iPad in your hands, playing with it--you'll discover quirks that only come from use, and the Internet community has been very vocal about them.
Charging (or Not) Over USB
Consumer Reports reports  (ha!) that charging the iPad over USB is, at best, hit-and-miss. Plugging an iPad into a computer's USB port, instead of an AC jack, often results in syncing only, with the battery refusing to charge. Initially it was thought to be some kind of snarky OS problem, since USB ports in Windows and Linux machines seemed to have the lowest success rate in charging, but Apple responded in the negative: it's a simple power issue.
The iPad requires 10W of energy to charge, much more than a typical USB port allows to trickle out. Many modern desktops, including Apple's own iMac, will have a "high power" USB port that can supply that kind of power, though it'll still charge extremely slowly compared to an AC outlet (some have said it's about a third as fast). That means most laptop USB ports won't charge it (one exception is Apple's newest MacBook Pro) and accessories like USB hubs and USB-sporting keyboards are certainly out of the picture. Good thing Apple included that impressive 10- to 12-hour battery life, right?
The Problem With iPhone Apps
Early reviews focused on the new, and by all accounts impressive (yet expensive) iPad apps. Why cover iPhone apps everyone already knows? Most reviews sufficed to mention that iPhone apps do indeed work, either in the "tiny picture in the middle of the screen" mode, and blown up to fit the iPad's bigger screen. But as it turns out, you'll mostly need to opt for those expensive native iPad apps--iPhone apps are usable, but you won't want to use them.
Gizmodo's John Herrman explains  exactly why these upscaled iPhone apps are a little bit...yeck. Text is a serious problem, blown up and blocky and pixelated, but that's not even the worst part. The problem, which somehow nobody realized until today, is that iPhone apps simply aren't designed for the bigger screen:
But the most jarring aspect of it all, as shown by the Tweetdeck apps, is that iPhone apps simply aren't designed for the larger screen. They look cartoonish. Text is massive. Buttons take up too much space. In the case of text-based apps, the iPhone-centric design is enough to render them useless. If an app hasn't been designed for the iPad specifically, you probably won't enjoy using it.
The one exception seems to be games, where the comparative dearth of both text and, often, buttons makes them less susceptible to Upscaled Disease. Maybe it's best to think about iPhone apps as a "just in case," rather than the core of the iPad's app catalog.
You Need a Case
This is a hard one to hear. Real gadget geeks hate cases. Modern gadgets, maybe especially those from Jonathan Ives's Apple studio, are designed to be as beautiful as they are functional. Putting gadgets in cases ruins the sleek lines and the careful choice of materials, and replaces them with a crappy $15 sleeve. Live dangerously, we say! The beauty is worth the risk!
But the iPad needs a case. It's too big to be protected by a pocket, and tossing it in a backpack is just too dangerous, really: it's too small to fill out any bag like a laptop does, and considering its most important element is a totally unprotected, 10-inch screen, it's bound to get scratched all to hell.
Even more, the iPad's peculiar design has led many to reason that this thing just needs a case. At 1.5 pounds, it's heavy to hold for long periods of time--many cases, including Apple's own , pack kickstands for superior viewing. And our very own Tyler Gray's adorable son discovered one more problem : the iPad's curved metal backside may be beautiful, but it's extremely unstable. It rotates and slips and slides and all in all makes typing altogether more difficult than it needs to be. So another lesson from today: Even case-haters will buy iPad cases, and given the first day sales , that means a whole lot of cases.