You can point to many moments from World of Warcraft's early history that define, comment on, or cement its status as the biggest game in the world at the time and a potent pop-cultural force. Its success was both immediate and gradual, which is to say that it launched big and then got bigger, at a rapid rate, for years. Its peak audience size didn't come until late 2010, six years on from its launch, but it was in the first three or four years that it seemed to loom largest in the culture.
One moment that sticks out for me was the launch of first expansion, The Burning Crusade, in January 2007. I remember covering the midnight launch at HMV in London's Oxford Street and being gobsmacked by the size and fervour of the crowd. This was something of the scale of a console launch - beyond it, frankly - only for an expansion pack for an online game. Not only were people there in huge numbers, but there was a specificity, an emotional tenor to their ardour that I would see on an even grander scale at BlizzCon later that year - something more tribal, more communal than the excitement of the console fans. These people belonged.
In a masterstroke of dramatic staging that must have been cursed by the studio's network engineers, Blizzard also gave the launch an in-game focal point with the opening of the Dark Portal, which saw players gather ready to step through the dread gate and enter a shattered new world. It broke the servers, of course, but even that became part of the story. Even while watching the login queues tick down, you knew you were participating in something huge.