I enjoyed Diablo 2's story more than it probably deserves, and Marius was the main reason. It might be an old tool to ground your earthshattering fantasy events by shackling them all to a ragged old man, but by God the contrast between that and D3's marvel superhero universe is stark.
You gotta have Marius, because nothing of what happens in the game is horrific if he's not there to be horrified by it.
That's why D4's direction seems promising. The drunken partying with slightly off-key violin into you getting slooooooooooooowly (very slowly - surprising patience in the cinematic direction for a modern aRPG) to your doom by the people you just saved stands out as a key memory, and anyone praising D1 and D2 writing over that scene is clearly playing favourites.
All D4's quest generally involves this theme; that the questgiver is lying to you about his motives and is actually a greedy bastard exploiting the other NPCs involved in the quest, and sometimes they actually make that fact cheekily apparant from the start. The one quest about the dude who gets flayed alive by a succubus and loves it is pretty cool, and there were lots of moments I actually remember despite only playing the beta once.
In contrast, I literally couldn't remember basic details like main character names after playing D3. It's funny that apparantly the old dude with the cool D4 intro, Lorath, is a character from Diablo 3 - I had no fucking clue despite playing Reaper of Souls recently.
HOWEVER, it's not all sunshine and roses. Chiefly, the characters kind of blend together despite them wanting Lorath to be that memorable main character besides the hero - the new Deckard Kain if you will - I doubt that will succeed. He's just a grumpy old man so far. While Vigo's story was tragic, he's certainly not a character I found myself forming a connection too. And the most concerning is, of course, Neyrelle, who is a better version of Leah, yes, but she's still Leah. A cheeky scholar-type go-getter has nothing to do in the Diablo universe. It reeks of Marvel and Whedon, and her characters undermines the horror they try to instill with her mother's fall.
So yeah, whereas I am extremely underwhelmed by the gameplay thus far, I am cautiously optimistic about world and story. I think they did a fantastic job with the aesthetic presentation, and atmosphere seems promising as well, even if there are concerns.