I got a few issues with how the story presented itself, to be honest.
But before I elaborate, I would like to say
SPOILER ALERT!!!The problem is that, some key points are not thoroughly fleshed out. The most jarring that I got is from Act II, end of Act III and the game ending itself.
Act II
» Click to show Spoiler - click again to hide... «
We know we are going to fight Zoltun Kulle, based on the achievements and leaks from website. What I am complaining about is the circumstances that lead to that fight. Zoltun Kulle found out black soulstone is infused with the souls of lesser evils, said to the player to abandon the quest, and then fight starts?
They missed an opportunity to actually forebode the events at the end of Act III
» Click to show Spoiler - click again to hide... «
Belial's identity. We all lead to believe that Hakan's action are just actions and such, and it was surprising to see that he is Belial. But what I was disappointed was how he reveals himself as Belial. He stood there, saying "Show me the soulstone!", then the player said "You will see it, Belial!". My feeling was "Yes, I didn't see it coming, but how the hell did my character know??"
Act III
» Click to show Spoiler - click again to hide... «
When it was revealed that Leah is Diablo, I was picturing her rage and sadness tips her over to the dark side, with a little push from Adria (for her "betrayal" part). The signs are there when Cain died in Act I. Quite disappointing they didn't follow this route, because it would greatly affect the characters psychologically, during the fight
Game ending
» Click to show Spoiler - click again to hide... «
Leah was not given a proper closure, other than "Her soul is lost" etc etc. Yes, that means she died, but for the love of God, she was there for three acts only to be discarded like that in the final Act? Again, they wasted an opportunity to make a better story, when they start with Covetous Shen's quote - "Let's hope her soul is strong enough" dialog. If they follow this line, it will make Act III ending a bit more bearable (in a sense she was turned to Diablo AGAINST her free will, as oppose to my idea that she herself turn over. Thus, it makes sense for her soul to fight Diablo from within as well, following Shen's lines).
» Click to show Spoiler - click again to hide... «
Our character is not given a proper closure. There should be an epilogue, about what our character is going to do past these events