The entire video was about Horde vs. Alliance! Did you not even pay attention? And the mage was fighting a summoned demon on top of a huge cathedral-looking structure, hurling fireballs and the such into its face. And at the end he cast Frost Nova and started to cast Blizzard, with a manic look in his face. Fucking awesome......
and you could see the pain and torture in the undead warlock's face, but grasping hold to the comfort that his demons would never betray him
I think the reason they changed the scene "right when it was getting cool" is to show suspense and make you want to play the game. I just don't see how anybody could not like the video. I guess if you were in a bad mood or something.