The game is so goddamn alive. I'm on 3rd mission of the Undead Campaign now and Arthas' arc had dominated the Human Campaign. I'm starting to lose my sympathy with Arthas, because he has no soul now. He just does what he's ordered. He's anything like how he was, but I don't think he's completely lost his soul yet. He still get surprised, feels things. IMO, even without the Frostmourne, he would gone evil. Not much as this, but still.
The characters were so realistic and you could feel that Azeroth was under big threat by multiple factions. Big conflicts were going on and these conflicts were essential in the story. These conflicts made the story alive.
Arthas was so cool. He was a hot headed leader who can give anything for his people. Perfect leader IMO. Until, he betrayed the mercenaries and lied to his troops kill them(Ship sinking mission). Of course I was shocked when Arthas suspends Uther from being a paladin. I didn't remember these things and thank god I didn't.
So, I figured out that Arthas has changed the whole world of Azeroth and there is no one has ever been who made as much as change as him. We need a new protagonist. Strong, good or neutral, badass protagonist. I wouldn't mind if he/she is evil. We have to like that character, that's all. Arthas was a lovable character, don't judge him when he was the Lich King. True Arthas was a paladin who sacrificed himself for his own people. Still, when he was a death knight, he made his best for his Undead people. He couldn't became the King of Lordaeron, but he became the Lich King. Sad story, but a good story.