Eye candy alone will not do it. I can go out and buy WindowsBlinds today and get all the eye candy I want. It's the extra little widgets and support built into the OS that will make it worth it.
On the surface, Windows 2000 to Windows XP upgrade was little more than just eye candy and changes to the interface. However, it was things like better device support (especially around USB device support), better laptop support (like better SpeedStep support, hibernate and suspend modes, etc), better wireless support that made it worth using.
The story will be the same for Vista... you'll see better support for multi-core processors, 64-bit processors, wireless technologies like Bluetooth, or whatever new technologies come down the pipe in the next few years.
Building upon a code base like Windows does not allow you to remove lines of code. There is so much code, and it is all interrelated in ways that no one person could comprehend that it would be too risky to start shrinking the code. You simply build on top of it.