Originally, Morrus started up EN World to provide a place for campaign material (if I recall correctly), and for users of Eric's site to have a place to play. When Eric decided to shut down his Rumors and News site, he agreed to coordinate with Morrus toward a continuity of experience, whereby users of Eric's site could have the same account name (and stats?) on Morrus' site.
As for me, I joined when 4E was nearly beginning, because EN World had effectively become a Rumors and News site again, but this time for the fourth edition. I wanted a forum site I could use without having to rely on USENET; Google showed me EN World, and I liked it enough to stay.
I don't remember exactly the first time I saw it - it was linked from many other sites and I visited it rarely on and off over the years but when the WoTC boards shut down I got on board the lifeboat and wound up here.
I was looking for information about the upcoming 3rd edition of D&D and found some guy named Noah's 3rd edition news site. I made a forum account (JediSoth). I went away for a while and somehow my account was deleted, so I rejoined after Gen Con 2003. I remember when it was switched to ENWorld, but I'm not sure that had anything to do with my account being deleted.
I'd been aware of ENWorld (and Giant In The Playground and one or two others) for some time due to mentions on the WotC forums. This would have been back in the 3.5 era.
I was one of the residents of the Off-Topic Tavern over there, and many of the other OTTers had been discussing a move to a different site when the Great Purge went down and a large number of them were banned from the site. I wasn't one of them.
I had made an account here and made a post or two before the Great Purge. Afterward, I posted here occasionally but didn't really become active on a regular basis until WotC announced they were scuttling their own ship.
Like others have said, I heard about 3E coming out and did an internet search in 1999 and found the old Eric Noah site. I was participating over at RPGNet and didn't become a regular here for several years, going through a couple login names, but then forgetting them. But yeah, I'm an old-timer: meaning, started reading the site before there was a forum.