One of the areas of Windows Vista on which we've received the most feedback is User Account Control (UAC). The Security Team has made a lot of changes since Beta 2 and created a video to show some of the main ways they have reduced the number of prompts in RC1. Check out this post on the UAC Blog to see the video and details on the changes they’ve made. My guess is that UAC is probably the area that has changed the most in RC1.
Sadly UAC still stinks for power users!
Most power users should run as a standard user most of the time and only use the credentials of their administrator account when needed. UAC in Windows Vista makes runnng in this more secure manner much easier.
If you are concerned about security, you should use up to date AV and anti-spyware software, use your firewall, have the latest updates, and run as a standard user. UAC makes this additional level of protection attainable.