I agree that closing cinematic looks a bit too "bright" considering what we remember from D1 and D2. Perhaps something more ominous would be more fitting. I do not however follow the rule that "dark" and "gritty" equals good. It is a matter of perspective and personal opinion but I like balanced stories. Too much of sugar or too much of "emo" makes an overkill. I can give you however that probably lots of people are expecting Diablo to be as dark as possible and I can uderstand those tastes. Yet I think the biggest success of this series comes from gameplay rather than storyline and by making the story a bit more "epic" Blizzard is just trying to fit it more towards general population.
Anyway, I cant judge the story as of now. Need to actually play the game, the cinematics alone are not enough.