Diablo 2 is a great game. They should have just kept the formula, then D3 wouldn't have been so fucking retarded.
Diablo 2 actually made significant improvements over Diablo 1. They were still the same core game, but they expanded out the player stats, skills, classes, and items. Diablo 3 was stale because it was too much like Diablo 2, only with souped up graphics.
Blizzards aversion to risk/sticking to the D2 formula/focus on shareholder needs made D3 a boring repetitive game, not any programmer with ideas that they could bend D3 towards more RPG elements. Making a game like D2 ten years after D2 came was a dumb idea, as most elements have been done to death. TO DEATH. There only innovation in D3 was the cash auction house. And because of this boring lack of innovation, the game sold a gagillion copies and pleased shareholders, but contributed nothing. I played through the game once and never looked at it again. I saw it all, and it was dull.
Boyarsky may have believed he could make something innovative, but just like any publicly traded company, risk aversion makes everything shit (
see: Starcraft 2)