Wrath of the Lich King has a lot of new dungeons, and hopefully you've been running them all as you level. Personally, I find them all pretty awesome, but Ahn'kahet: The Old Kingdom is the first one that completely blew me away. Overall, I'd say it's not as good as Halls of Stone or Halls of Lightning, but Ahn'kahet has certain elements that make it one of my favorites.
Not only is Ahn'kahet visually stunning, but it gives you a glimpse into the story of Yogg-Saron and his relation to the Scourge and other forces. It's a little repetitive in some places (the first boss is a bit boring), but it also houses some of the best 5-man bosses we've seen yet. The final boss, Herald Volazj, is the embodiment of the technical advances Blizzard has made with the gameplay in Wrath of the Lich King. I almost feel bad mentioning him at all, because I've been running Ahn'kahet repeatedly with people who have never done it before. Seeing them wig out the first time they do the boss is awesome.
I don't want to spoil the mechanics of the encounter, so I won't. Saying something is going to happen is kind of a spoiler in itself because saying "Hey, watch this!" sets off all kinds of alarms, so it'll dull the edge a little. I think it's worth to make sure people actually do this dungeon, though. Do it! Go! Right now! You can start doing this dungeon at level 73, maybe a little earlier if you have good gear from The Burning Crusade.
You guys that have done Ahn'kahet, back me up on this one. Herald Volazj, one of the best 5-man bosses we've seen yet? Yes? No?