These reviews of Warcraft III (many of which are already linked to 404 pages), Blizzard's last pre-WoW game, tell the story: Blizzard was already known as a master game studio, renowned for their polish. Critics called them a relatively conservative developer, taking old ideas, rather than crafting their own, and shining them until they sparkle. Their releases were few and far between, but always worth the wait (come to think of it, that's not too different from nowadays). I was working at Gamestop as a manager when World of Warcraft was first announced, and I remember the reaction among gamers as puzzlement: Warcraft III had just released, and it was amazing, in terms of both sales and gameplay. Why did Blizzard want to go back into the Warcraft franchise, especially with some weird subscription-based model, when they could be working on another Diablo or Starcraft?
The beta eventually quelled those concerns, though -- I remember it was tough to get into at first, and I actually ended up watching a Ustream of video from the game (a gnome leveling through Dun Morogh, I believe it was -- you can see early gameplay trailers of the game over on Blizzard's site, and that's about what it looked like). They eventually opened up the beta, and word of mouth started spreading from there: this was a different MMO from Everquest or Dark Age of Camelot, two of the popular MMO games at the time. This was an MMO done right. Instead of waiting for five minutes after a battle, you could just eat food to regain health and mana. Instead of losing levels when you died (or sitting there dead for hours just waiting for someone to come along to rez you and save the penalty), you could just run back to your corpse and keep playing. Instead of having to all be on the same quest, you could just share quests, and so on.
Here's a very early preview from Gamespy describing both players' puzzlement at Blizzard's direction right back into Warcraft, as well as how different the game would be from existing MMOs. You can see how they come at it: earlier MMOs were slow, plodding, complex, tactical affairs, while World of Warcraft came along and showed everybody how fast and fun an MMO could really be.
And another preview link from IGN, this one talking about what players experienced in the closed beta. Keep in mind how early this is in the game's development -- for a long time during the beta, hunters had no talent trees or pets, and many of the game's current features and systems (PvP honor, battlegrounds, endgame raiding, token-based rewards) would only show up much later on. There were almost no addons or any of the convenience features that we got through the patches. The game these people played was vastly different from the one we play today: it was focused on the early levels, offered questing as a main content feature, and was largely unfinished and unexplored at endgame.
The early game launched big, but the big launch was always tagged with the qualifier: "for an MMO." Back then, MMOs were for hardcore gamers -- people who played with an Internet connection and wanted to spend $15 a month on a video game. But WoW drew a bigger audience than most MMOs, and copies of the game were even hard to find for a while. Blizzard's supply exceeded demand on the realms, too -- while the game did start up with 41 servers (most MMOs then made do with half of that), players had a tough time staying logged in. Pretty much everyone agreed that it was a good game -- when you were actually logged on.
Still, the official reviews (many of which have actually been added on to that page since launch) were great. While there were certainly issues with the game, most everyone saw it as a revolution in MMO gameplay -- a revamping of what these subscription-based games could be, and a chance for almost anyone to create a character and play with their friends online.
While it took a long time for World of Warcraft to build up its legendary audience, and even longer for other developers to see what Blizzard had done (take a hardcore passion and put it into a form where even casual players could find enjoyment), it all started five years ago on Blizzard's original launch. Those early days were definitely marked with excitement. Right after the announcement in 2001, we all wondered just what Blizzard was thinking -- why do Warcraft again in such a weird form? But clearly they knew that there was something to this MMO stuff, and five years later, we're still playing.