For the longest time in vanilla World of Warcraft, players were content with collecting the dungeon set 1 pieces from Stratholme, Scholomance and Blackrock Spire. A purple item was a incredibly rare thing to see in the early days of WoW. It was a status symbol, a badge of pride for raiders who managed to band together to defeat Onyxia or brave the depths of Molten Core. With epic items soon came epic discontent, largely from casual players who either didn't have the time or the inclination to raid. As time went on and more raid dungeons were released, the complaining continued; players who were unable to dedicate the time needed to successfully complete a 40-man raid dungeon felt it was unfair that they could not obtain epic gear.
It was a valid complaint, but it took well over two years before Blizzard finally did something about it. About four months after the release of Ahn'Quiraj, Blizzard implemented Patch 1.10, "Storms of Azeroth." Among the fun changes like the introduction of weather in Azeroth and quest-experience-to-gold conversion at level 60, casual players finally received what they'd been asking for: a quest line that didn't require raiding and would allow them to upgrade their dungeon sets one into new ones that included epic gear. This set is called, appropriately enough, dungeon set 2.