A friend told me about the site back in 2001, she was posting some and I got on, created an account, then promptly forgot about it. I wasn't trying to lose weight at the time, so I didn't look around too much. Fast forward to 2006, I had lost about 75 lbs and was trying to decide if I wanted to keep losing weight. I hadn't started with any particular goal other than wanting to feel better. I remembered this site - well, kind of remembered it, I went to 2fatchicks.com instead, which used to be and may still be a porn site.
After my misstep, I found the right site.
What really drew me in was the unwavering support. No backbiting, no snarkiness but real support and encouragement. Oh yes, like mango683 mentioned: the ticker - I loved getting to move my ticker (ahhh, the good old days - I haven't moved my ticker in ages).
I stay because I truly enjoy being here. I feel like I have broadened my horizons in more ways than just weight loss. Some of the smartest people on the web hang out here.
I also love that there is an active maintainers forum. People who share their experience of maintaining weight loss for years. I don't know anyone in real life who has done that.