Let's face it, Diablo 3's story telling is HORRIBLE. I don't think I have EVER played, heard of or seen a triple A title with just such absolutely terrible story telling.
The foreshadowing, James Bond Villains with even less depth, and worse of all completely underselling the big twist made for just bad story telling all around.
But note, I haven't mentioned anything about the story itself. Why? Because, if you take out the terrible script, the story wasn't actually half bad. The harkening back to D1 showing that your hero was Leah's father and how coniving Adria is were pretty interesting plot points. If they could just update the script or simply write better for the expansion I feel like the game itself would be a lot better. I've done multiple play throughs of Warcraft III and Starcraft II and each time I gladly watched the cinematics again and followed along with the story. Diablo 3 has the potential for that as well.
Diablo as a series hasn't really been focused on story; but Blizzard attempted to focus a bit more on it this time around and half-assed it, as such it is their duty to ensure they at least do right by it if it's going to be included in the game.
So do you folks think that it's possible for Blizzard to rectify this in the expansion? Honestly it would take just a few tweaks here and there to fix D3's own story let alone doing it in the xpac.