I wish I could remember what got me to MeFi, but I can't. Surely some other site linked to some interesting thread.
The realization that it's a real community
The terrorist attacks of September 11 happened, and MetaFilter had the news, links to real stories of real people and what they were doing, thinking, feeling. The comments and links were amazing. That's what made me want to join, because I felt the need to participate, and wanted to be part of the gang.
MeFi has the funniest, nicest, not-so-nice, smart and real membership. As many MeFites have commented, I start too many sentences with "As my friend online said." I don't mention that the person online is named Madamejujujive, or Lipstick Thespian. I'm thankful to Matt for having the idea, and to Matt and the burgeoning crowd that make it work, and the individual people who add the bits and pieces that make it exist.
"We have cameras" is still one of the funniest things I ever read.