I found it linked to from some other site, whose name escapes me now. I was a reader for about two years before joining, which was very nearly the day I discovered that the $5 registrations were open. (I admit, I still have a grudge that they were closed for so long, or rather perhaps that there was a secret sign-in page that proles like me didn't know about. Hmph.)
It's the people
My favorite part of Metafilter for so long has been, yes, the high level of intelligent snark around the place. Sometimes it gets a little strong maybe, but there's always someone on hand to remind everyone when it's getting too mean-spirited (I've probably been guilty of that a couple of times, admittedly). And many times the snark is dead-on appropriate and howlingly funny. Whatever obnoxious package of talking points is making the media rounds, I can count on a MeFite to dissect it and show just how the levels of public opinion are being pulled. I count MetaFilter an important part of my media education process.