In college in 1998 I worked at the IT helpdesk. As you do, I got interested in slashdot. From there, I found Rusty Foster's site, kuro5hin. Kuro5hin was much different in those days than it's current incarnation as a troll hideout, but Rusty had mentioned Metafilter a time or two on the site. However at the time signups were closed. So no account for me and I mostly ignored it. As Kuro5hin was starting to turn into it's current mess, many of the users were looking for an alternative. At the time, Metafilter signup was re-enabled, yeah! but the five dollar fee stopped me in my tracks at that point. A year later, and links to metafilter from TheophileEscargot on Hulver.com and I completed the signup process and have been a background commenter / lurker since then.
Internet Detective Squad cases
Whether it being discovering givewell founders recommending themselves to themselves via askme, or fake's friends getting helped in DC / NY, metafilter figures stuff out and helps people out. That's pretty awesome for such a large community site.