They killed Deckard Cain because they wanted to own the storyline. And there's no better way to sever any links between the old and new content than by killing characters who a) had been through it all and b) players like. DC was an inconvenient personality for the world they wanted to run. It barely fucking makes sense that his niece talks about "stories" while the man who lived through all that shit is standing right there in front of her. He should have clocked Leah in the mouth with his walking staff the minute she started referring to him as a delusional old man.
So yeah. The character who reads a lot of the found journals was/is intended to be Cain's replacement in some form. From long-winded old white man to a younger and slightly preening Middle Eastern scholar.
Tyrael fits that description too, except he wasn't as well fleshed out as Cain and got his own, preposterously out of place story. On the one hand, while it's good that players do most things in the story, the story ultimately isn't about us. D3's story is really a story about its NPCs. Tyrael "rediscovering" him, Leah finding out she's a demon bebeh, Adria turning out to be a secret bitch, Haedrig, the Followers, Zultan Khule.....Act 3 is the only act where it's about the players, getting shit done, instead of waiting for NPCs to open doors, hit buttons or discover themselves.
D2 felt like a player marching into ancient evils to discover and destroy it. There's rightly a sense of "Who the fuck are you guy--ARGH" from the bad guys, because you were really just adventurers.
D3, it practically feels like fucking prophecy due to how much exposition there is, and how many "Gee whiz!" moments are written into the story. The players just happen to be Nephalem coming back into their powers, and you get called out by everyone. King Leoric happened to come back because Tyrael fell from Heaven smack dab in the middle of the Cathedral, and another Butcher demon happened to show up? Zultan Khule was teh uber Nephalim people considered as bad as Diablo, and he just so happens to have possession of the Black Soulstone? Adria has been playing the long game since D1 Act 1?
It's all written like an overblown movie, which is why characters talk so fucking much. A story that was completely driven by our characters, without a small hamlet worth of NPCs and flunkies trailing us, would have made it feel less like we're being led by the nose by the story.
I wonder if there is a curse of three when it comes to long running series. Matrix 3. Doom 3. Diablo 3. Quest For Glory 3. Battlefield 3. Max Payne 3. I could probably sit here for an hour and rattle off titles that really weren't all that great compared to their predecessors, because the designs fundamentally altered the formula in some ways that made it less enjoyable. Maybe the curse of three is when any series reaches high enough levels of success that it starts to impact a thing in many ways: stories get more "epic" and self-serving and gameplay mechanics get upended in the name of The Vision for breaking with the past.
There was a time when I excused a lot of the changes to stuff because every developer has a prerogative to tweak, refine and improve a game. But after taking in the complete package, and Jay Wilson running his mouth, it really seems like D3's intent was to be everything D2 wasn't. Where it was muted, D3 colorizes. Where it was open-ended enough to allow someone to cripple a build, D3 steers the whole course of your development to 60 for you. Where the story was relatively hands off, D3 demands you sit through yet bit of plot development, and half of the game is running back and forth to satisfy it. Adventure Mode will be a god send, because it will finally strip the story out of it so you can actually feel like what you're doing is what you want to be doing, not what some NPC told you needs done.