In retrospect I have no idea how we could ever find vanilla WoW fun. Compared to its expansions, the vanilla experience is just so... hostile to players. Grinding was absurd, only a small number of players actually got to see the best content and a number of anti-frustration mechanics we take for granted now simply didn't exist back then - in fact, the developers back then argued against a number of them, I think.
Perhaps it's just that it was a fresh experience for most of us at that point. To me the golden age of WoW will always be patch 2.3, when my guild managed to make some excellent progress in raids. At least, that's what I used to say. These days, I have no idea.
I just really liked Outland. It was an alien place that was beautifully designed and always had something novel on the horizon. The Burning Crusade was such an enormously differently designed game compared to vanilla - players were given meaningful context for the quests for the first time, and you no longer felt like you were some sort of second-rate hero who was given the jobs nobody wanted (poop collection quests aside). Dungeons were no longer a strange collection of dead ends and demi-bosses, but were challenging, tightly scripted events with mechanics that made sense.
In short, The Burning Crusade was the point where the developers got a handle on what it meant to design a game. Vanilla WoW had some neat ideas for its time, the most important probably being always offering you a new quest, but it came with a lot of luggage likely inherited from earlier games in the genre, and a lot of the design choices were nonsensical as the developers hadn't figured out what worked in this type of game. Interestingly, I've seen a lot of single-player RPGs released later take a few leafs out of the WoW book with regards to keeping your game systems clean.
But look at me ramble. This thread is about Mists of Pandaria, not poorly structured essays about key moments in MMO history. I've pre-ordered the game and will be picking it up in a store tomorrow. At this junction I'm past WoW - you can only play the same game for so many years before you get bored of it. For the sake of nostalgia, I will be giving the expansion a shot for a month, possibly two, but after that I doubt I will be back until it's time for the next expansion. After seeing what they did to my beloved Shadow spec - gutted, hamstrung! - I feel little desire to invest myself in WoW again to a meaningful degree considering how I'm enjoying the heck out of TOR at the moment.
Now, the only question is, do I go Troll or Pandaren Monk? Once you go Troll you never reroll, but...