I was under the impression that vista's code was a hell of a lot more manageable than xp. In fact, I was under the impression that that was the primary purpose of vista, reign in the windows code base to something modular and easily maintainable, and then just throw in some token efforts to encourage people to "upgrade to it" when the majority of the benefit exists only to Microsoft's own devs, meaning that end user benefits of vista won't really manifest until the next MS OS.
Vista is supposed to be much more secure though, but considering that many of the exploits, viruses, and spyware that worked under xp still work under vista, I don't take their claims of security very seriously.