Way back when, Apple brought the GUI to the personal computer and to the masses. They didn't invent the personal computer. They didn't invent the GUI. They did make a great product that set the direction of human-computer interaction for the next few decades. Apple wasn't able to capitalize on what they accomplished and are now somewhere around 5% of the PC market share.
Apple didn't invent the MP3 player. They reinvented it by making it easy to use and cool. They now own the portable music player market. Apple didn't just reinvent the way we listen to music, they reinvented the music market place with the iTunes store.
Apple didn't invent the mobile phone, nor the PDA. They didn't invent the touch screen. They did put them together in a way nobody really expected. (This is starting to sound like a 3M commercial.) The iPhone is extremely elegant and easy to use. It set a new model for human-computer interaction, one dictated largely by the size and power constraints of the device. No pointer, no windows, no drop-down menus, no multitasking, limited typing. The result is a set of task-focused applications that are pretty darn intuitive. It's not how we were used to interactive with a phone, nor a desktop or laptop. In many ways, it's better. Apple took the opportunity to simplify for the better. No file management, minimal configuration, no confusion about how to install or remove applications.
The iPad is just a big iPod Touch. That's disappointing in some ways. But it's also what makes the iPad awesome. To appreciate this awesomeness, one first has to understand why tablet PCs have sucked. The big reason: Windows. The problem with Windows (and Windows apps) is that it wasn't designed for use with the constraints a tablet imposes; it is ill-suited for use on a device without a pointing device, keyboard and relatively large screen. The same would be true of Linux/Gnome/KDE or Mac OS.
The physical constraints of the iPhone required the iPhone OS designers to rethink how to build applications. It turned out these simplified, task focused applications were really easy to use. I found myself using Safari, Mail and Facebook on my iPhone even though my MacBook was sitting right next to me. There are many tasks which do not require the complexity of desktop applications, but do need more screen real estate than an iPhone. There has also been a shift over the last 15 years or so from using computers for content production to content consumption. This is where the iPad fits perfectly. You're not going to write a textbook on the iPad, but you may read one. You're not going to use Photoshop on the iPad to edit a photo shoot for a magazine, but you may view it with the iPad. The iPad will not replace the desktop computer and everything it does, but it can replace it for what most people casually use a desktop for. Apple successfully reinvented the tablet computer.
Whether or not this version of the iPad is wildly successful is irrelevant. Apple will inspire copycats who will take the design and build on it, as Google has done with Android. This will make Apple up its game. Everyone wins.