Eurogamer has a nice long look at the early days of World of Warcraft, way before Northrend and Outland and even Molten Core, back when the question wasn't just how big the game would get, but whether Blizzard, a company known for their polish rather than their size, could pull off an entry in this new MMO genre. They've interviewed some of Blizzard's luminaries, and the piece offers a really good look at what it was like at Blizzard even before WoW's release, when they were hashing out some of the ideas and mechanics that have now set the bar with World of Warcraft: the stylistic Warcraft look, and questing as storytelling (originally, they thought they'd only do quests through the starting levels, and then have the game move to a grinding, monster-killing stage towards the end, but players said the game was boring without quests).
There are all kinds of great little tidbits in here: originally, Warcraft III was planned with the over-the-shoulder look that WoW now has, and that's one of the reasons they wanted to create a more straightforward RPG game. Tom Chilton showed up on the team about a year before WoW's release, and to his surprise, the game was almost completely unfinished -- the level cap was only 15, the talent system wasn't implemented, the AH or mail systems weren't in, PvP wasn't in at all (of course, even at release it was pretty barebones), and endgame raiding was nonexistent. Most of the things we think of as intrinsic to the World of Warcraft -- even things like the Horde and Alliance not speaking to each other -- were debated and almost not in at all as they moved towards release.