Its what is called progress, when XP first came out it was bigger, had more bugs, and was more expensive than the ones before it. The bugs in Vista are rapidly being sorted, support for Vista is going up more every day. Vista supports more system memory than XP did, you can do alot more on Vista (if you know how), the graphics are better quality. In short everything goes up, software requirements go up, hardware goes up, and to go with this the operating systems increase in all respects as well.
Plus as deeyan said, (or attempted to) microsoft will stop supporting XP, as has happened with so many operating systems in the past. This will leave you no option but to progress with the rest of the world.
However in saying all this I do admit that Vista is about the most controversial new OS that Microsoft has put out, and untill I got Vista on my new laptop and learnt how to use Vista I would have agreed with you. As Vista, as well as being the most controversial is also the most different OS Microsoft has ever released.