Hidden behind all of this hoopla, however, is the fact that as much of an improvement Vista is over XP, its main competitor, Mac OS X, still stacks up really well -- and even tops Vista in several important areas.
There are nearly an infinite number of ways to compare complex beasts like operating systems. I'm going to skip low-level issues, like comparing driver architectures. Most people just don't care about things like who has the superior kernel. People care far more about the parts they see and work with, so that is what I'm going to deal with here.
While Vista is indeed a major update to Windows, there's a lot of it that is, quite frankly, just Microsoft making up for lost time. The last non-server release of Windows was in 2001 with Windows XP, with only a single major interim update in Service Pack 2. In the same time, Apple has been steadily releasing updates to Mac OS X on what was a yearly schedule, now around every 18 months.
This means that while Mac OS X has been steadily evolving through 10.0, 10.1, 10.2, 10.3, and 10.4, and is now working toward 10.5, Microsoft was waiting on what would become Vista. When it was obvious the original Longhorn OS wasn't going to happen, it took the Windows Server 2003 code base and used that for the basis of Vista. It also chopped quite a few features out of Vista, most notably the WinFS object-based data storage and management system, which had been promised in various forms since the first blurbs about Cairo in the early 1990s.
Microsoft had two serious issues. First, it had to make this update of Windows revolutionary enough that it came close to justifying the delay. Second, it had to come up with something that would stand up well with its main competitor in the desktop OS market, Mac OS X. Has it succeeded at both? I'd argue that the former's almost a nonissue: Vista will sell well, because the world won't have a choice. As far as the latter, well, probably, but you'd be hard-pressed to say Vista's better than Mac OS X.
In a nutshell, Vista vs. Mac OS X is Revolution vs. Evolution. It's about a massive, long-delayed upgrade that has to account for almost six years of progress by its competitors, versus a well-executed strategy of regular updates. While updating an operating system is never something that can be called easy, Apple's strategy has been the better one for keeping its OS on top of things, something Microsoft has admitted to in a roundabout way.