User Access Control is a good feature. Prohibiting implicit changes to administrative settings is good. Linux has done this for a long time with sudo. But Vista made a good thing annoying.
You have to click Ok every time instead of it realizing you know what your doing. Sudo can remember for a certain amount of time. Although I complain, I can actually live with this downside.
It is ugly and slow. I have a year old machine, that I think is fairly beefy. Even still, my entire screen turns black for a second while the UAC dialog loads. Very annoying.
Elevation is a pain. If I am running a program that needs to do administrative tasks, I usually have to restart. Yes, certain applications elevate easily, but Powershell and Visual Studio don’t. Why should I have to run them as an admin? Can’t I just elevate permissions as necessary?
I think that UAC solves a security problem but fails in the usability department. I love the concept. I love Vista. I haven’t had a chance to play with Windows 7, but I really hope they improve on some of these areas. Preventing malicious activity is a great thing, it just can’t hinder normal activity.