Plotlines

A core conceit of the move to Northrend was the Horde and Alliance taking the war back to the Lich King and it was rather assumed that the assault would culminate at with the attack on Ice Crown at the end of the game ... and in one fell swoop (albeit one that was telegraphed)  that premise was shattered and a new one takes its place. The fragile cooperation fostered by Jaina and Thrall is gone, the grief and drive for vengeance after Angrathar (to give the gate its proper title) and Putress' betrayal of all three factions (Alliance, Horde and Scourge) to serve the Burning Legion completely changes the dynamic and literally puts the War back into Warcraft.

That Blizzard do this isn't a great surprise, but that they do it before the halfway point of the game is (Andro was 74 when she completed the quest chain almost exactly a week after Wrath launched) is, and that while you get to witness the betrayal at the Wrath Gate via cinematic, the characters are key participants in the diplomatic and military actions that follow. Its an excellent use of the phasing system (I'll put up pre- and post- shots of the Wrath Gate, as well as the move involved follow up once I figure out how to minimize spoilerage).