Diablo is the biggest offender, but he's just demonstrating a pattern that all the bad guys share.
"YOU WILL NOT DO THIS THING BECAUSE YOU ARE PATHETIC AND WEAK." (Find the Sword Fragments, Defend the Keep, Destroy the Siege Engines, Destroy the Hell Rifts.)
"YOU DID THAT THING DESPITE BEING PATHETIC AND WEAK, BUT IT DOESN'T MATTER. MY NEXT CHALLENGE WILL STOP YOU." (The whole fucking game.)
"OK, THAT CHALLENGE DIDN'T STOP YOU EITHER BUT I WILL KILL YOU LIKE ANTS WHEN WE MEET."
"ARGH, I'M DEAD. NNNNOOOOOOOOOOOOOOOOO, HOW DID THIS HAPPEN."
Azmodan and Diablo are basically the same character. They say the same basic shit, have to constantly reframe what you're supposed to care about because you steamroll them and they both talk in that throaty, unimaginative demon voice.
I mean, compare D2. "LOOKING FOR BAEL?" That's the only fucking thing Duriel ever said and it made him a character people will never forget. (That and because he's a total cheap bastard.)
You know what D3's writing reminds me of? The Power Rangers. Some fucking exec was like "We cant' make this too subtle, otherwise we can't get legions of 12 year olds on board." Nevermind that D1 was basically the poster child for adult gaming and was boycotted as such. And yet D3 is filled with impaled corpses, exploding bodies and the like. You know, kid stuff.
That's what's so irritating. They knew what was expected of them, they put the barest framework in there, and then they a) went apeshit with all the lore that happens
in the books written after D2: LOD and put it all into game and b) they boiled down everyone to hammy caricatures. You can't even fucking tell the differences between the Lords of Hell if you were just listening to their tone, because they all sound the same: angry and feckless.
Also, the sheer amount of homages to the original game. Why the fuck is Leoric back in the first place? WHY THE FUCK IS THE BUTCHER HERE? IZUAL, YOU'RE FUCKING DEAD AND HAVE BEEN FOR YEARS, GO AWAY. I can appreciate Act 1 trying to tie back to the original game. But they went completely overboard with it. I'm still surprised I didn't have to kill Griswold AGAIN, or that the Spirit of Wirt didn't come back to be like "Hey, remember me from that game you loved!??!"
But the absolute best example of the poor quality of the writing is....the guilt spirits in the High Heavens. Nowhere does the logic of the writing fall apart more than when NPCs you didn't like try to make you feel guilty for shit the story did, and not you. Zultan Khule is trying to make me feel bad about him dying? Did the people who wrote the dialog even
fucking play the game or listen to what someone else wrote? Maaaayyyybeeee if Zultan Khule had been like "We had a chance to stop Diablo in his tracks but you killed me, so all this tragedy is your fault", I'd have been like "Maybe." But then again,
Zultan Khule attacks you in the first fucking place. I can't watch those bits without having this running through my head:
http://www.youtube.com/watch?v=gjTdO5bNHj0And remember, these are the Lords of Hell, masters of whatever domain they claim. And they can't even manage to guilt trip effectively, let alone scare, intimidate or cow you. And that thinking is written into the character dialogs, where every single character heroically tells each bad guy NPC to go fuck themselves.
The core dialog and banter is basically what killed the entire story. If they didn't talk directly to us, and we didn't directly talk to them, I probably would have found more personal meaning and context in what was happening.