Raiding has been the generic end game for massively multiplayer online games for the past 10 years. Originally comprised of hard-to-kill, non-instanced world and dungeon bosses, end-game raiding tested players' coordination, skill, communication and tenacity. World of Warcraft pioneered the accessible raid -- instanced dungeons that guaranteed loot drops. Many people forget that guaranteed loot drops was a huge deal, right along with no failures during crafting.
Vanilla WoW raiding was an evolution on the EverQuest system, naturally, due to the prevalence of EverQuest players' not only designing and producing World of Warcraft but also their prevalence in the installed player base. Raiding had a language all its own. The first expansion to World of Warcraft: The Burning Crusade, attempted to stretch the bounds of raiding by scaling down player numbers and, at the same time, creating new and unique challenges in an attempt to make content more accessible. EverQuest routinely failed to make content accessible, and WoW was determined to turn the tides with the introduction of the 10-man raiding tier comprised of Karazhan and Zul'Aman. The popularity of 10-man raiding soared more than Blizzard could have ever imagined.