Wrath of the Lich King saw the introduction of several elements that furthered the incorporation of lore into the game. Phased quest chains allowed players to actually see their effect on the zones. Cinematic cutscenes made it feel like you were playing through a movie. The faction leaders of the world were suddenly far more active than they'd ever been before. But those were the major, blowout moments that made the storytelling work. What most didn't quite recognize were the subtle efforts of the lowly NPC.
In classic WoW, players literally had to walk up to NPCs and speak to them to engage them in conversation. In The Burning Crusade, that changed slightly -- NPCs now recognized players as they walked by, according to their reputation. In Wrath, suddenly NPCs were not only recognizing players, but they were whispering players, recognizing players. Prior efforts by a player were acknowledged, even if it was just a simple "I remember you."
What Wrath of the Lich King began was a revolution in WoW gameplay that would spin into full-out overdrive with the launch of Cataclysm. The lowly NPC was no longer an unimportant figure; he was a comrade in arms, a fellow hero, or a taskmaster -- and he made certain to let you know it.