Sorry Steve, but if you can find only one that means that you have not spent too much time in Vista. To me it was easy to answer:
1-Because the handwriting recognition is better, it's so far the only one that learns from you getting better everyday. And no in the other way around where your UMPC is teaching you how to handwrite so it can be recognized.
2-Because of the Gestures, you don't need to install anything, they are there, 8 easy gestures that allows you to navigate, copy, paste, cut, undo, etc.
3-Because it was designed thinking about Touch Screen adding new features like the Touch Pointer. For the first time you have a tool that allows you to use your finger as a mouse, but not just that. IT'S EASY TO USE! Because it was designed taking in consideration that is going to be used with your finger.
In less than 5 seconds I got the answer.
Vista is the best OS for devices with Touch Screen, XP Tablet PC Edition 2005 does not come even close. But not just for Touch Screen, in General, Vista is a better OS for Tablet PC devices.
Once you have your unit working with all drivers configured and working properly Vista is as stable as XP. So far the only issue that I have seen and experienced is the one mentioned already by many other pages, and it´s the one when the HDD goes crazy doing something in background. But this is not something that happens all the time and that takes a lot of time. At the beginning I had the same feeling than JK thinking that was going to crash but now I got used to it because it never crashed on me. The only time when I got some blue screens was when I was betatesting the HID drivers.
I'll never go back to XP in any UMPC-Tablet PC.