I found AskMe by googling for some information; I read part of the archives and left, but then the same thing happened again and again. I had become a regular lurker and the only thing holding me back was having to create a Paypal account... which turned out not to be necessary after all.
tl;dr: Turned right at Canada
The change of the site's culture through the sexism threads
I love a lot of aspects of MeFi: the smart people, the bookish content, the niche specialties and interests, the snark, and the personal anecdotes. However, what turned me into a convert was the introspection of the community and the resulting change in the culture of the site after the sexism threads. 'The unexamined site is not worth visiting', as a smart fellow said, and MeFi, imperfect though it may be, sets a bar that few other websites can jump over, and has adapted to the needs of its members to create a healthier environment. I was glad to see that the 'community blog' really had a community: Happy birthday, MeFi!