I just got back from a 3 week trip where I left my laptop behind, and only took my iPad (and my mobile phone).  Spending more time with the iPad has convinced me that it represents a sea-change just as important as Windows was in the early PC evolution.  What makes the iPad such an interesting new platform?

1. Form factor - This is the most obvious difference between the PC platform (Mac and Windows) and the iPad.  The fact that you can hold the screen in your lap and interact with a touch-screen interface.  Microsoft has proved that this is NOT sufficient, as we've seen a lot of failures in the tablet form factor for Windows-based machines.  But Apple's implementation of a touch-screen computer is particularly simple and elegant.  It is a pleasure to use unlike any other PC experience I've had.
2. Application Security  - I've been hearing more and more that iPad users are installing and using MANY more applications than Windows or Mac users.  When I reflect on my own behavior, I realize how hesitant I've been to install Windows applications for fear of corrupting my system (crashes, slow-downs, security holes).  The way iOS manages applications it effectively creates a sandbox so your apps can't really damage your system as a whole.  And the single-application running at a time, ensures that your system resources cannot be consumed by background applications running without your knowledge.
3. App Store - The most important aspect of the iPad platform is the iTunes App Store.  The iPad would not have been successful without the previous success of the iPhone.  Not only did it solve the chicken and egg problem of having a large number of apps available on launch-day, but it also had several years of building up a developer community that was familiar with building apps for the iPhone/iPad platform.  Because Apple manages the distribution and payment of applications, App developers have a frictionless means of selling their applications to users.  It boggles the mind that Microsoft did not long ago develop a similar system for Windows.  Perhaps they didn't think it was necessary, since anyone could distribute Windows apps on their own.  But they did not count on the perceived complexity and trust issues that would prevent most users from being promiscuous purchasers of Windows applications.

But there is a dark-side to the Apple-owned iPad/iPhone platform; the fact that it is controlled by a single company which exercises strict control on entrance into the App market and protects it's own competitive interests by rejecting applications that they themselves want to monetize (they have already had the US Justice department get involved in anti-trust complaints, such as when Google tried to deliver it's Google Voice service on the iPhone).

I'm looking forward to an Android-based tablet, and the introduction of another App marketplace being run by Amazon.  That will give developers a great choice with more than one option for application distribution.

I also think there's an opportunity for the HTML5 platform to evolve into a competing API/platform for application distribution.  Web apps will evolve to have all the capabilities of the iOS platform (offline support, local storage, rich graphics (SVG + Canvas), camera and microphone support, etc.).  But we still lack an App Store equivalent for web apps.  Google announced one around Chrome at Google IO in 2010 - perhaps we will see them ship it with the introduction of Chrome-based tablets.