The User Account Control is one of the most hated features of Vista, constantly throwing up pop-ups asking if you really want to do what you just asked to do. In an intriguing defense, Microsoft says they know it's annoying -- they did it on purpose.The UAC warning goes up whenever an application tries to take administrator privileges. UAC is Microsoft's way of trying to beat developers into writing applications that don't make quite so many requests for admin privileges, which are unnecessary.

Ars Technica:

Microsoft's approach to UAC is a carrot-and-stick way to get developers to adopt Microsoft's latest views on secure application installation and setup, but it does come at the expense of the user experience. It's hardly no surprise, then, that one of the most popular post-Vista install activities is disabling UAC. I still haven't disabled it myself, but I've come close. Microsoft claims that 88% of Vista users leave UAC enabled, and that application developers are already greatly enhancing their setup routines to avoid process elevation.

By the way, did you ever notice that, in religious arguments about operating systems, the Vista guys always say, "Vista isn't so terrible! You stupid Microsoft-haters are making it sound worse than it is!" Sure, they're passionate about defending Vista, but they never say, "Vista is great!" they just say, "It's not so terrible!"

Disclaimer: I've never used Vista myself; I'm a Mac guy.