The advent of achievements changed the game of WoW in many ways. Not only do they provide recognition for time and devotion put into the game, such as the Loremaster or Justicar achievements, but in Wrath content and beyond, they can actually change players' approaches to game encounters. Consider the meta achievements for the Wrath raids. Who would have actually tried downing Sartharion with all three drakes alive if there had been no reward? Achievements prompt us to try more daring or difficult strategies for boss fights, do crazy or counterintuitive things in PvP combat, or run headlong into live mine fields. They also allow Blizzard developers to work in hard modes for certain bosses, giving them a scaling difficulty that provides accessibility for casual players and challenge for the hardcore.
So what about the vanilla and BC encounters? With a few notable exceptions, classic and Burning Crusade content has not received the achievement treatment.
If achievements had been part of WoW from the beginning, what would they have been like? Which crazy boss strategies deserved their own achievements in classic and BC encounters? Have you ever done something awesome in older content and thought, "Man, that should have been an achievement"?