I think it is the challenge and surprise gameplay that had such an impact.
How many times do you die playing D2R for the first time? In D2R you fear every single room inside a dungeon(you dont even have the option to choose a difficult level).
In D4 how many times did you die in the campaign? (i played the hole game at the highest difficult possible and didnt died once).
My point is that in D2R you feel like a human opressed by demons, in D4 its like a walk in disneyland.