One thing that was missing from the Wrath expansion, conspicuously or not, was a world boss on par with those such as Kazzak and the Emerald Dragons. For better or for worse, Blizzard completely removed them from the game, reserving the massive figures towering over the landscape for instanced quests or dungeons.
Blizzard touched a bit on why they've not used world bosses in Wrath in the recent Twitter developer chat. They observed that there are issues with the bosses that have prevented them from feeling comfortable making more world bosses, such as issues with groups racing to bosses, or the ability of people outside raid groups to deliberately mess up boss attempts. On the other side of the coin, though, they did imply that they like world bosses, and will probably implement more in Cataclysm if and when they can find a satisfactory work-around for the problems.