I've been diving into Windows 7 lately (Release Candidate 1, Build 7100), and it's my first real introduction to User Account Control on my own equipment. I've been running XP on my primary desktop and laptop, having let Vista pass me by, after demoting my Steve login account to a Standard (not Administrator) account, UAC gave the most bizarre prompt:
You can click around on this all day and never get anywhere: there is no place to type the password, and it completely boggled me for a time (it might invite you to use a smart card, but that won't have a place to enter a password either).
In my zeal to run as a limited user, I neglected to first enable the Administrator account before downgrading my own. Admin is disabled by default because the user created during installation is an administrator, and this reduces the attack surface some.
So I had essentially locked myself out of my own system, having no accounts that have working Administrator access. That part is my own fault, but the UAC prompt surely did not help me: asking for a password with no place to enter it does not seem like a good experience, and I suspect many will blame UAC instead of themselves for this.