Honestly, I don't really find UAC very intrusive anymore.
I'm not a fan of the double-prompting that happens sometimes though, especially on file operations. I also find the background flickering very distracting (caused by a display driver issue), so I disable Secure Desktop, but not UAC completely.
The problem is that when you're first setting up Vista, you're busy installing all your drivers and software, and since most of these operations alter important parts of the system, you see the UAC elevation prompts coming up very often.
Once you've got everything set up, you shouldn't really be seeing elevation prompts all the time though.
1) If you're being prompted often when performing file operations, it means that you're trying to change something in a directory that you don't have ownership over. If it's not an important system directory, consider changing its ownership. I took full ownership of my entire second physical drive because it only contains my own user data. No more UAC prompts for file operations unless I'm touching system folders.
2) If you're trying to change a system setting (launching regedit, for example), and you see a UAC prompt, this is good behavior. This is what prevents malware that's running under your credentials from making damaging changes to the system without your consent. In XP, everyone ran with admin privileges, which was perhaps less "annoying" to some extent, but also far more insecure. Most people really don't make system changes all that often though, so as I said, after the initial setup is done, these prompts should become less frequent.
3) If you see a UAC prompt when you launch regular applications, it's happening because (a) the application needs to access a system directory, the HKLM registry hive, or make some other system-wide change for some legitimate reason, or (b) the application is poorly designed and is probably trying to write user data to HKLM or Program Files when it should be using HKCU or AppData instead. In the latter case, send an email to the software vendor and ask them to fix it. As Vista goes mainstream, more software vendors will start doing things the correct way, rather than the sloppy way, which XP quietly allowed them to get away with.
Besides these three broad scenarios, you should not be seeing elevation prompts very often if you've set up things properly.
