Even though some critical features (player-versus-player battlegrounds, for one) weren't implemented, the game clearly outstripped established rivals in polish and scale at launch, and demand was so intense that Blizzard's overloaded servers spent the first few months fighting a losing battle against the tide of players.
So you can hardly call Blizzard's first steps in massively multiplayer gaming tentative. Nevertheless, WoW in its early days was a shadow of what it became over the next five years, in terms of the quality of the game itself as well as the size of its player-base (two million in June 2005, 12 million today) and its impact on popular culture. You can trace that journey in the media: in the news and in reviews (Eurogamer'soriginal 8, the re-review 10, the reviews of the Burning Crusade and Lich King expansions). Best of all, you can hear the story of its development in the words of its developer, in parts one and two of Eurogamer's exclusive Making of World of Warcraft.
But some of that story can only be understood, and properly put into context, in hindsight. You'd need a book to comb over the significance of every event, so below we single out one development in the game and one in real life ["IRL", surely -- Ed.] for each of WoW's five years of operation. Some of them are famous, some obscure; it's not a definitive list and they might not be the single most important moments in WoW's history, but they all say something about where the game has been, how far it's come, and how much it's changed the gaming landscape.
2005 in-game: Corrupted Blood
It did. On some servers, players exploited teleportation to carry Corrupted Blood from Zul'Gurub's remote jungle location to the game's major urban centres. It spread like wildfire, killing lower-level characters in seconds. Regular gameplay was completely disrupted; some players took malicious glee in spreading it, while others formed an impromptu relief effort, taking it upon themselves to heal the sick. Eventually, Blizzard was forced into a hard reset of affected servers to kill it off.
2005 in real life: Leeroy Jenkins
Caught on video and posted mock-seriously on WoW's forum, the moment spread first across the game's community and then beyond it, and eventually escaped the confines of the internet and into national and international media. Leeroy became a sort of folk hero of WoW. His name and idiotic feat are immortalised as an in-game achievement and title; his image is recreated alongside Warcraft's heroes and villains in the WOW trading card and miniatures merchandising; the player behind him, Ben Schultz, does public appearances at conventions.
2006 in-game: Shadow of the Necropolis
2006 in real life: Make Love, Not Warcraft
To see the rest of the list, head over to US Gamer.