Well, WoW -- happy birthday! I don't think anyone, least of all Blizzard, expected the game to be anywhere near as popular as it's become, and it's just getting stronger. The game has changed a lot in the past four years, both for better (if you're me) and for worse (if you're a forum troll commenting on every change that has ever been made or ever will be made, unless it results in their character being absurdly overpowered, in which case any effort to fix this will be termed "a slap in the face"). Classes have also drastically changed in the process, with far less attention being paid to their traditional roles in previous MMORPG's and more attention paid to soloability, adaptability, and just plain fun. The character you rolled when you first began playing may bear no resemblance to the character you expected to be playing now, even if it's the same toon. And for some of you, that you're playing an MMORPG at all would have been a laughable notion before WoW debuted.
If your main's been the same from the start, how have you seen it changed by the game's advancement? What was the single biggest change for your character's playstyle, and did you agree with it? If you didn't, did you switch mains? Personally, I've had the same main, a Tauren Druid, since my first day, and I think most longterm Druid players would point to the meteoric rise of feral (and more recently, balance) in early Burning Crusade as being a sea change for the class. Most early Druid players would have scoffed at the notion of a Druid doing anything for a group that wasn't healing -- or, for that matter, being a good PvP class.
And did you remember to log in and get your baby Blizzard bear?