I disagree with all my heart.
If you bought the original Diablo, it came with a booklet of a very nice mythology. It was a dark story, it was gothic and horrific and it served a pretty kickass background to what you could find in the game. Few things were cooler than reading about Lachdannan or The Warlord of Blood and then actually meeting them in the game. It made you feel like part of an evolving epic.
Diablo II was less complex, but I won't argue very much; the story was very well presented through Marius' memories and I didn't see that, in the end, Baal was the one under the shroud. The story really served as a continuation to the themes from the original tale.
Diablo 3 IS light on story. The twists were sort of predictable and nearly every boss you find is either dispatched too quickly of a 2D character that taunts you endlessly until you kill him.
Come on, Blizz. You're better than that.