I think a lot of people had connections to Arthas / The Lich King. You played 3 campaigns as Arthas (1 human, 2 undead). You knew from the start of WotLK that this was going to be the last boss (that dragon instance doesn't count) which builds up even more, by the time we defeat him - rather than "Whoa hey, suddenly this guy is the villain of the expansion?" He watched you all this time, you saw him in several zones (As Alliance when you're in the spirit realm in one quest, you can find him. And if you go near him, he kills you.)
Then for me it's also the sense that it's easier to 'relate' to him, or at least he feels like a villain that doesn't seem like a 'I can destroy planets' danger, but enough to kill us off (which he did). To me, any boss or villain that is powerful because... they were born / made that way, is extremely boring. Arthas was just like us once, adventurers - and then all these things happened which led to this. He wasn't born with a great destiny, he was just a boy, who became a prince wanting to save his people - and things went wrong.
This is why I wish something could happen to our "power level" and reduce us to mere adventurers again. I want relatable villains who are not world threats anymore - to me, the Lich King was the most powerful I felt comfortable with. After that, things just got silly.
Wasn't the whole point that the Lich King wanted us to fight through all he laid before us to become as strong as possible, then in ICC as we fight him, kill us and raise us as his most powerful servants? If he really just wanted us dead, then I'd agree.