This is kind of unsexy, but I found MeFi via StumbleUpon. I happened (well, stumbled) upon a post about cover songs and clicked the links, read the comments and kept coming back to the curious blue site with the intelligent, if overly ardent, arguments. I discovered the green shortly thereafter and I've been hooked ever since.
You were doing it wrong
MetaFilter clicked for me when I was explaining something to my sister and she asked me, "did you read about this on Ask MetaFilter?" Why yes, yes I did.
This little family of websites is my favorite place on the Internet. When I tell people about this place, I want them to love it like I do and I get huffy when they don't. (Savages.)