From my point of view, the whole siege idea looks written by a person who didnt even have a clue about warfare or even history.
Azmodan has to lose, eventually, or else we wont have a nice ending. But the way he loses is too much stupid for the way he is supposed to act.
A couple of mistakes i noticed besides what everyone has said already.
1) History has shown that fortified structures provide a huge advantage in a defense. The only efficient way to siege a fortified position is wearing out the structures with constant siege weaponry, cutting supplies, and forcing the enemy to come out or die locked up in is own stronghold. And NEVER, NEVER, go full frontal assault while the defenses are still up, even if you have ten times the number of units than the enemy. The guy who wrote the script didn't even read about the battle of troy, one of the most known. The greeks had to finally resort to the horse idea because after days of sending thousands of soldiers to die they couldnt take down the walls.
2) The whole act is a constant win for the hero, we never lose a battle, we always go forward and crushing stuff. In real warfare, sometimes you win a position, and sometimes you lose it. The whole act looks like the hero started at the worst moment and just went up from there to the end, with no drawbacks, no bad situations, not even thinking if he was going to lose, because he is actually steamrolling every single issue that comes at him.
Yes, the keep is breached, yes, the soldiers are slain, but.. you just keep winning like those things are just "acceptable losses". I wish there was a moment in the act where tyrael would say "!@#$, we are screwed, what do we do now?" at least for a minute.
Picture helm's deep in LOTR, Theoden is soo convinced he is going to win the first 15 minutes, he boasts that the keep in unbreachable, a minute later a suicidal uruk-hai crushes half the wall of the side of the keep by using a nicely placed powder keg, next thing you are seeing is Theoden's 'ooops' face, and running for his life. Bastion's keep's walls seem made of adamantium... because they can withstand the fire of siege weapons during the lighting of the signals, the raising of the catapults, the whole time spent on finding the breach, and the time till you destroy the ballistae. Seriously?
Only those two things added to not telling what should we do everytime, and the siege would have looked more real.