One of the major things that vanilla World of Warcraft struggled with was a compelling story. Yes, we had a particularly epic tale spun with the release of Ahn'Qiraj and an even greater tale with Naxxramas. But while Onyxia, Molten Core and Blackwing Lair had stories that were interesting enough, it was difficult for players to pick up on those stories and follow them in a coherent fashion. Ragnaros in particular had a story that was entrenched in several different leveling zones as well as a few instances.
Meanwhile, the zones that you encountered from level 1 to 60 by and large didn't have a coherent story to tell. Each zone had little tidbits of story here and there, but nothing seemed really dire or important beyond a few epic, sweeping quests. As for faction leaders -- well, they did very little beyond sit in their capital cities and occasionally send players on errands. This is something that has continually changed and improved with every expansion that has been released. The story in WoW has never been as accessible as it is now.
But Mists of Pandaria has the potential to completely blow everything before it away.