IMO the sense of wonder and the defined dark aesthetics of both the worlds created for Diablo 1 and 2 are something Blizzard will have to proove it can still pull off.
D3 was initially also developed by Blizzard North. But then it got overhauled, multiple time. I'd say Diablo 3 became a good game... but it started out terrible. The RMAH has taught me a great thing though (something I should've learned by playing MtG back in the days).
I mentionned that Diablo 3 feels different. Its art style and gameplay loop are both going in a different direction. It's quality has nothing to do with it.