UAC is not smart in any way. It doesn't try to discern something that might actually be a threat. It just throws up a prompt about something that might conceivably be exploited. It also doesn't ever relax. You could click the System Control Panel (also called Advanced System Settings in some areas of Vista) 75 times in a row, and it would prompt you with the statement "Windows needs your permission to continue" every time. So basically, it adds an extra click to the process of accessing this tool.
There is nothing inherently wrong with this approach, which has been used by other operating systems before. It's not a new idea, and it's not a bad idea. But the devil is in the details of how it's implemented. As a Johnny-come-relatively-lately to the security bandwagon, Microsoft has embraced security principles fervently. What that means is that, if there's even a small chance that opening a settings dialog box, starting up an applet, or running an installation program could present even a slight security risk, Windows Vista is going to prompt you with some sort of UAC dialog box asking for permission to proceed.
This is a short list of just a few of the processes that require confirmation to initiate:
Opening Disk Defragmenter, System Restore, Task Scheduler or Windows Easy Transfer
Adjusting font size, connecting to a Network Projector (opens two dialog boxes in succession) or accessing Remote settings
Opening these control panels: Add Hardware, BitLocker, Device Manager, iSCSI Initiator, Parental Controls, Advanced System Settings, System Protection or Remote Settings
Additionally, many processes that don't prompt you at launch, such as Windows Defender, Windows Firewall, Ease of Access, Internet Options and a long list of others, do require your permission for specific settings.
Taken one by one, most of the processes that are gated by UAC seem very reasonable. Microsoft rethought a great many restrictions that made little sense between Vista Beta 2 and RC1. But taken as a whole, UAC is going to seem like a burden to many users who are tired of Microsoft and other software makers protecting us from ourselves.
Proponents of UAC claim that after the first several days or weeks after Vista is first installed (or you receive it on a new PC), the experience of constantly being confronted with UAC dialogs slows down. But for some people, UAC numbness creeps in quickly. How long before they stop reading the prompts or considering what they mean and just click OK every time? It can quickly become muscle memory.
The average Vista user will have little idea about the rationale behind UAC prompts. To that person, UAC may seem scary at first but quickly became a petty annoyance. How long before people realize they can turn off UAC in the User Account Control Panel?
This is the worst problem about UAC. Has Microsoft overbalanced it, and turned it into something that will actually defeat its purpose? There's a very real possibility of that.