I think the big difference between the launch of the iPad and the iPod & iPhone, which also happens to be the reason why the tablet launch is such a big deal to Apple, is that this is the first major product launch where Apple does not "only" have to unveil a great product, but also tell us why, exactly, we need it. In other words, not only must they provide the answer - but the question, too.
If we go back to 2001, the situation is different. Looking back, we often focus on strictly the portable mp3 player market, and we make it seem as if Apple kick-started that market. In all fairness - they probably did. However, can you really classify the portable mp3 player market as something separate from the portable personal audio market?
The reason I'm asking this is simple: the iPod did not have to create demand. The iPod did not have to ask the question - it "merely" had to answer them. The portable personal audio market was old, and huge; it was kickstarted by Sony in 1979 with the launch of the Walkman, which was so incredibly successful that "walkman" became a generic name for portable cassette players.
Everyone had a portable personal audio device, whether it be cassette, CD, MiniDisc, or even DAT-based. When the iPod came onto the scene in 2001, it didn't have to ask the question - Apple already knew well and good that people wanted portable personal audio. What Apple needed to do was investigate the existing market, and design a product accordingly. It did so, and it did so well.
The iPhone was launched in pretty much the exact same way. The mobile phone market was already completely saturated by the time the iPhone arrived in 2007; just like 6 years earlier, Apple already knew that people wanted mobile phones. Everybody already had a cell phone. Again, it was Apple's job to analyse an existing, saturated market, and come up with a product accordingly.
In both cases, the question didn't have to be asked. Apple "only" had to provide the answer. The reason I'm putting the "only" between quotation marks is because I do not mean "only" in the sense of "that's easy".
Fast-forward to today, and the tablet market. Quite bluntly, there is no tablet market, and there never has been. There is no market, and there's no saturation. As such, there's no existing market, filled with happy and/or disgruntled customers, for Apple to study and learn from.
It's been a while, but the iPad is the first major Apple product in a long time where not only does Apple have to come up with a great product, but it also has to explain to us why we want it. We already knew we wanted a good portable personal audio player, and we already knew we wanted a good mobile phone.
Now, the real question here is: has Apple succeeded in asking the question? Has Apple's presentation yesterday made people go, "Darn, I didn't know I needed that!"? The unusually underwhelming response on the internet - even on Mac forums and blogs - leads me to believe that no, Apple has not succeeded.
Now, this doesn't mean the iPad won't sell, because I'm pretty sure that it will. However, had Apple succeeded in making us go "Darn, I didn't know I needed that!", the device would've been a surer hit than it will be now.