Diablo 3 will be a fail, because by the time they started to create it, they used a rendering tochnology form way back and, it took them 2 years in wich software renderings have evolved way better, example: Assassins creed 1 medium graphics on my machine around 25-30 fps, Assassins Creed 2 with visible improved graphics full detail maxed out 35-40 fps.
It will happen as it did with starcraft 2 the more human players on the map the less fps because its cpu rendered, AI players are gpu rendered better optimized, ancient technology, they fixed that isue with multyplayer games long ago