the main difference in previous Diablo games was, that even if the game had barely any story, it was ont force feed to you, each time you open a "game". Particularly when playing it in multiplayer.
I mean it makes you think about it what their idea was when they decided to make it like how it is now, in an game about EXTENSIVE farming ...
Or why the bosses are all "instances" and not open, like how it was in the old Diablo games, which somehow gave the game a feeling of playing in an yeah, open world. When ever you reach the end of the dungeon in Diablo 3, the gaming asking you "are you sure to get in there?" it somehow breaks the wall, disturbing you while playing.
The main problem I have with the plot of Diablo 3 is the fact that the characters talk to much, that the story is trying to be way to personal, about this one nephalem, trying to save the world killing everything that was evil.
The hero in Diablo 1 and Diablo 2, was a "mortal", nothing more then one character of many. Most of the deamons and leaders would not even see him more then as a fly. Even after you killed Mephisto, Diablo would not do much but saying something like "Not even Death can save you".
And now compare that to those endless monologues from Assmoda, Diablo and Magdha. You might realize that when it comes to the story of Diablo 2 for example, it was Decard Cain that filled the role of the main quest giver supporting you on your quest. While there was not much talking, the characters you had in the game did their job very well.
As you say, the game was not so much about the story, but it had its own charme. Diablo 3 might not be TO far behind the other 2 games, but the fact that you have to hammer on your space bar so many times because you heard the lines they babble now a thousand times ... makes it really bad.