I think the biggest difference between D2 and D3 is that D3 is missing David Brevik, Matt Uelmen and Blizzard North.
this....... times 1000
Plenty of arguments can be made over gameplay options, and character building ect.... and to be honest i do like the freedom that D2 allowed, but D3s gameplay doesnt really bother me so much, as i am enjoying it a fair amount atm..
but in the end, the absolute biggest offenders are
-horrific, campy, dialogue
- entirely forgettable soundtrack
- Over all lack of grimness / macabre
- Story more so with atmosphere and setting (see below)
Diablo 2 was very ambient, there were plenty of undertones that told the story of the game without forcing atrocious dialogue exchanges and set pieces down the players throat.
It maintained its grizzly and mature atmosphere while letting the player experience the game, conveying only the most minimal story line information through lore excerpts, and small, well voiced exchanges of small bits of information from the various npcs, piecing together a bigger picture, without all the nonsense that is present in D3 (diablo and azmodan's constant reminders are one such example). Its all literally shoved in the players face, and executed so, so poorly.
The game just over all doesnt really take the player seriously... and as a result just completely and utterly looses feeling that was received from diablo 2 and even 1...
Diablo 3 lacks all the subtly of its predecessors, and none of their grim, macabre atmosphere and near immaculate soundtracks, that captured the mood to a perfection.