After the defeat of Arathas the Scourge were wiped out and the threat of the Lich King was removed.
A close second was Deathwing as a manipulator rather than ROAR, BURN, DESTROY.
Me, I expected that they'd get rid of the Scourge/the Lich King (fighting Undead was getting pretty old), but keep Arthas the character.
I don't mean redemption.
I mean, all that set-up they made about how he really was pure evil, and committed to it - and how his level of commitment was his true strength (that questline in Northrend - you control Arthas as he kills all of his men, and the ghost-child tells you that his true
strength wasn't in being powerful enough to do it - but in actually going through with it). He wanted that power, deep down. If he hadn't, he never would've been able to "usurp" Ner'Zhul instead of ruling together.
HE would be King. He worked hard for it, struggled, and, in the end, prevailed.
... What would be a worse fate for someone like that than to take it all away? He'd end up pathetic. Pathetic, desperate, dangerous - hated by the world, but, as ever, determined
What would he do after? If he'd managed to escape without his sword and helmet, would he try to get his power back? Would he try to get some NEW power? There'd be many, many, many people hunting him - but he is a dangerous man even without Frostmourne.
Would we hear about him working behind the scenes? Would we also occasionally work with
him when a situation came up that threatened the entire world (and by extension his utterly hopeless plans of regaining his power)?
But then it turns out there must Always Be a Lich King - and he was somehow irredeemable but still holding back the Scourge at the same time.
I really can't understand it. "The Lich King" wasn't that important. It wasn't something that had to be preserved for the sake of future storytelling. Preserving the Black Dragonflight for the future was a good idea that's turning out interesting. This was not. They've done several new stories with Scourge remnants since - but there's neither hide nor hair of the new Lich King. His existence left a lot of people seriously unsatisfied with the ending, and now he is in cold storage just in case he's ever useful again.
I sort of thought they were of the same mind when they made it clear that WOTLK's Lich King was Arthas
, and not Ner'Zhul. They could've went with an "Arthas is gone" angle in WOTLK, or stuck with that silly two-people-talking voice they used in the original Naxxramas. They didn't. The character was who mattered.