Vista is a major rewrite. Not a total rewrite, but then again it didn't need to be like OS X did.
Think about it... total replacement/update of the following:
Printing subsystem, display subsystem (DWM),
graphics subsystem (DX10, DX9L), Media subsystem (Media Foundation), Audio (Core Audio), Media Center, Networking, brand new Explorer, new search system, new IIS7, system-wide RSS, new color system, new image codec system, new backup system, new memory subsystem, new speech system, text rendering, encryption, UAC, slideshow, .NET 3.0, new power system, clickonce, WIM, Powershell (not technically in Vista but released at the same time), Hypervisor/virtualization, 64-bit.
On the surface the improvements are minor but under the hood they are huge.
Most of those aren't rewrites. A lot are completely separate products which were BUNDLED with Vista. Those can't count. IE7, for instance, has only minor improvements, and (at least on XP systems) has a tremendously slower GUI. .NET 3.0 is also a bundled feature (though it is integrated somewhat into Vista.) There was a 64-bit version of XP, so that is not a new feature. DX10 is simply another update to DirectX.
XP -> Vista is perhaps like Panther -> Tiger (though the end result is still less than Tiger). Vista was SUPPOSED to be somewhere between that and OS9->OSX. But they had to cancel that. Now it is supposedly Vienna which will do that.