Check out 43 Things, an addictive site from the Robot Co-op (also an all-Mac company). Its brilliant design shatters some omnipresent myths about user interface and design.
Myth 1: Users need explicit instructions.
This site will not be conceptually familiar to most users. But by encouraging experimentation (you're not afraid that by clicking a link you'll "break" the site), it allows users to learn by doing rather than by reading.
Myth 2: Give everything a name.
Nothing on this site is named with jargon. To go to your list, you just click on "your list"--none of this "Online Goals Chart" or "Dynamically Generated Interest Repository" nonsense.
Myth 3: Establish a clear hierarchy.
What is the "home" of this site? Is it your personal list? Is it the page you enter on? Does it even matter? No, not really. The age of entry pages is almost over. Soon the ubiquity of Google will mean that we find the page we want, not the site we want (and then OWL will mean we get just the piece of information we want...but maybe I'm getting ahead of myself). More power to the user...

Legacy Comments
I find that there's a dangerous amount of zealotry creeping into these posts. Hopefully she doesn't know what that means...
Nice anaylysis of the site, though.
The trick will be to use the natural weight of items from honest users and collaborative filtering to drop the zealotry pieces "off the charts." Of course, it's difficult to promote popularity without stifling serendipity, but it's not impossible. They might be able to pull it off....
What is so bad about this post??