World of Warcraft launched in 2004 and became the game that defined what an MMO was for an entire generation. At its peak it had over twelve million subscribers, which at the time made its population larger than most countries' capital cities.
What Blizzard got right was accessibility. Everything before it demanded enormous time investment before it became fun; WoW let you log in for an hour and accomplish something. The quest structure, the class fantasy, the sense that Azeroth was a place you lived in rather than a level you ran - all of it became the template.
The Horde and Alliance split gave the game its cultural identity. Two factions who could not speak to each other, which turned a design limitation into decades of genuine tribal loyalty.
The raids are the other legacy. Forty people coordinating for months to kill something is a social achievement more than a gaming one, and the friendships that came out of it outlasted the guilds.
This collection covers Azeroth's factions and iconography.