That said: there's some major story fail here. Spoilers abound ahead. Blizzard writers - I know you probably can't change these things at this point, but you REALLY should. And I would love you forever if you do. You don't have to take my solutions. But seriously, anything would be better than what you did.
This is long, but here are my problems with this section of the story. And how I would fix them. I hope some of you will take the time to read this. I think I have some solutions that both greatly improve the story and are more familiar in tone to the previous games.
The portrayal of Belial and Azmodan:
One thing that the previous Diablo games never failed to do was to make our major enemies intimidating as all hell. You run into the Butcher or Andarial for the first time, and you crap your pants. To say nothing of actually meeting Mephisto. And Baal. And DIABLO.
Belial and Azmodan fail to be memorable villains, despite fun boss fight mechanics and design because they don't live up to the character traits the writers gave them. Belial is the Lord of Lies, but he fails utterly at lying. Azmodan is a master strategist who makes every cartoon villain mistake. What? You had awesome character concepts that you just didn't use. These are the Evils that united to defeat the PRIME EVILS. They should be bad dudes.
Well Belial is the Lord of Lies. So shouldn't he, you know, actually be remotely good at lying? His identity is so transparent that even your character is just like, "dude, you're not fooling ANYONE." He should be the most devious bastard in the franchise history. How can I take the physical embodiment of lies seriously when he couldn't fool a child? His fundamental problem is that it takes the player almost no effort to deduce that the child using supernatural powers is the greater demon himself. The Lord of Lies biggest lie is that he's a good liar. Wait, that was a pretty good lie...
Everything is the same till you get to face Belial. You approach the Child Emperor, and he morphs into Belial. Just as you suspected. You expected more from a greater demon. You're geared for your climatic boss fight. Belial cackles at you and gloats that the city has been his all along. His minions uncloak all around you and swarm to attack you - you were geared for a boss fight, but now you're fighting a dangerous mob as well. Undeterred, you charge through his minions, and with all your strength bring your axe down on Belial's laughing face.
To your surprise, his body crumples instantly - that was the easiest Lord of Hell you've ever fought. You feel victorious for only a second - even before his body hits the ground, you know there's something wrong. And Belial's form slowly reverts back to that of the child emperor. It was an illusion, and you killed a kid - His guards that you killed - people too. They must have seen you as the same demon you thought they were. An astral projection of Belial appears, laughing with glee. "Sorry, you just missed me. If you'd like to pay a call, please look me up at Mount Ariat. Ask for the Lord of Lies." And laughs and laughs and laughs. Yeah. You're stunned. And ANGRY.
The Iron Wolves burst in and see the carnage you've wrought. The charge you, and you are given the option of slaughtering your fellow humans - ones that helped you just a quest ago - or running from the city in shame and defeat. Act II, done. But you don't feel victorious. No climatic boss battle. You've killed a kid. And innocent humans. You did the job of a demon for him. Vengeance will be yours.
And this game just got more personal than ever before.
Wall of Text Break. Azmodan's problems and fix will appear in the next post. Feedback is appreciated.