It's so long ago, I honestly can't remember (my first userid was in the mid-three-digits). I'm tempted to say it had something to do with Jorn Barger, but I don't trust my memory on this - and God knows I'm not braving exposure to all of that madness again to find out for sure.
Every time I think I'm free, it pulls me back in!
MetaFilter, I just cain't quit you... And God knows, I've tried, twice: once on September 12, 2001, for over a year, and again in November of 2004, when Kerry lost to Bush, for another two.
Why do I come back? Because as exasperating as this community can be, they've proven they are best able to dig up and deliver fresh, interesting content long before it appears on any other human-curated aggregation site. Maybe it's just the breadth of the population; maybe it's because my personal interests intersect with a lot of people here - but I find myself following links on about 75% of what's posted and digging down into the comments on at least half of those.
This is why my "fondest memory" isn't actually a specific incident, or link, or funny branding tag ("MetaFilter: We Never Let You Go.") - it's the accumulated knowledge, entertainment, and simple pleasure I've derived from everybody else's hard work. Even when I reach a saturation point, I always seem to come back, recharged and ready for more...