Or should I say, Undead? From my experiences, WoW is best when it involves the Undead.
The Plaguelands are underrated. Even Blizzard doesn't seem to fully realize what they started there. The cauldrons, countering the Plague, that's huge. That would have been a good setup for a backstabbing by the Royal Apothecary Society, who while they are fanatical about fighting the Scourge, they might be afraid of the side-effects of the reversal of the Plague, such as their destruction/death.
The Eastern Area has a lot of wasted space, which is good. It forces the player to see the scale of the devastation. It gives plenty of room for all sorts of quests. Plagued trolls, the remnants of the high elves, there's a lot there, and it's all mixed in with the Undead.
The Ghostlands are creepy. Very creepy. The entire quest area is amazing, with a perfectly timed rep scheme, and a build up to a final attack on the traitor.
Scholomance is a great instance. It's semi-linear, you can't really get lost, so it would fit well with the BC design. Still, it's complex enough to not seem like a glorified hallway. The pulls are varied, the bosses have some interesting mechanics. Even after the nerfs/adjustments, it's still a place that can kill a bad player. The quest chain from the ghosts outside sets a perfectly awful tone.
Stratholme: I admit it, I'm amazed that it has managed to burn, unchanged, for 3 years. I know WoW is significantly after TFT, but maybe it changed since then, as players we can only comment on three last three years that we've seen. The quests are a little strange and seemed concentrated on the Scarlet side, but still, they were more interesting than "kill X number of Y." Oh right, looting the head of the Grand Crusader is a... surprise.
Naxxramas I'm talked about before. I actually did it Saturday night, had a blast again. Sapphiron has a really cool animation, neat mechanic with the ice blocks too. Though the pre-KT trash is really annoying.
Northrend, it's filled with the dead. It's the throne of their King. How can it not be amazing? Yea, you might have noticed that I'm really excited.