But why are you comparing a game released in 2012 to a game released in 2000?
Honestly, vanilla d2 wasn't as bad as d3 is because people didn't expect d2 to be that good after release of d1. D2 ,however, has set a certain benchmark of what diablo games should be like, and a game released 12 years later miserably failed to achieve that.
As a standalone game, d3 would be okay, as a continuation of the diablo series it's a major step backwards.