A friend sent me a link to a thread about an article that mentioned a band I liked, and a few users on Mefi were ragging on the band. Thus, my first experience was a classic Your Favorite Band Sucks moment.
I had stumbled across Metafilter on rare occasions before that and it had registered as that "oddly colored discussion-board site". After spending time reading that first thread and poking around the site, I decided I'd check back in a couple days. After lurking for two months, (which included the epic Sketchzilla thread), I couldn't resist joining.
Percy, the Human Skull
There's something about a bedazzled human skull that tickles me. When a user asked the community how to donate or dispose of an inherited human skull with a scarab named Percy, I found it hilarious. Then, some weeks later, another user asked about disposing another bedazzled human skull, and I thought "OMG! IT'S PERCY!" Was it the new owner having doubts about their new acquisition? As it turned out, the first user's spouse didn't know about the original post, so they both thought the community of Metafilter would have all the answers about human skull clearinghouses.