Storm Peaks are the best imo. Pretty zone with awesome music, a lot of titan buildings and of course Thunderfall <3
But Icecrown is great too, it definitely is special and it actually does have a feeling its controlled by Lich King, with Scourge completely overwhelming anything there.
Overall Northrend >>>>> Outland. And what i like about Wrath is how you can see that Lich King's plan actually progresses, that he is ACTIVE, keeps on strengthening his forces and "recruiting" new races. In TBC Illidan was, well. He was mainly known from TFT, there wasnt almost any character development inside a game. He was mentioned, but he hardly did anything. In WotLK right off the bat Lich King is introduced as the main villain of the expansion (I know illidan wasnt in TBC, but still, as the lord of outland he had far too small role in the game). He isn't sleeping. His forces awaken Vrykul, cutscene in UP, Angrathar, amazing Drakuru betrayal chain. Additionally some character development for example in Matthias Lehner chain. You can feel actualy presence of the Lich King in the game.
And ICC is
HUGE, unlike this stupid excuse of a fortress BT was
PS. Bugged Honor among Thieves ftw! On first day of testing it at Patchwerk 6.5k dps, yesterday on 10man Loatheb - 6k dps. Fun times.