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

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??"

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

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).

Our character is not given a proper closure. There should be an epilogue, about what our character is going to do past these events