Whole story of D3 is based on black soulstone and fact that Diablo planned to be contained in it since day 1 (awakening in Cathedral in Tristram). But that just doesn't work anyhow. Chronologically (roughly):
1. Diablo and his brothers has been captured by Horadrims with soulstones. Diablo has been hidden inside Horadrim Cathedral. OK
2. Somehow either before his capture (I'd like to note- he's been imprisoned for few ages or more) or after he managed to contact with Adria and managed to convince her to fulfill his plan.
Something's already wrong here isn't it? Either Adria is about 1000 years old or somehow Diablo managed to summon her while being imprisoned few hundred meters under ground. Even if somehow he'd managed to set all this in motion he would have to actually know about black soulstone. But how if Zoltan created it after imprisoning brothers? But that's not all.
3. Aidan and Adria land in Tristram. Adria aids Aidan in defeating Diablo (and puts soulstone in own head).
Ok- so Diablo planned to free himself and then get killed? So why was he so tough? Also, Adria gives not a single hint that she's got a plan to help Diablo. She's mysterious, yes... But apart from fact that she appears in Tristram when everyone else flees, she seems totally honest. Is she embodiment of Belial or what? Damn- even he wasn't especially cunning, and she's playing wildest trick in the world- sorry Sanctuary. What would have actually happened would every single adventurer fail? It's not easy to kill lord of terror after all. Better yet- why try to flee and take own form?! He took over whole kingdom with ease, would he want, he could actually do whatever he wishes.
4. Aidan- currently Dark Wanderer- goes to the east... Creating quite a mess on his way. On his way he frees Baal and Mephisto. Mephisto is killed and Diablo is killed as well...
AGAIN. It's quite risky plan when it involves TWO deaths... And again- what if adventurers would fail in their quest?
5. Baal reaches mount Arreat and corrupts Worldstone.
It's a tricky one. Is he some kind of loose end in Diablo plan? Throughout whole D2 story I seemed for me like brotherhood had some kind of plan involving all three of them. Baal even succeeded- partially.
So what if he wouldn't be killed and his sould wouldn't go to black soulstone? It seems like serious overlooking in plan- maybe Diablo should make sure that Baal dies before him for sure?
6. 20 years passes... Another overlook. To set things in motion, Leah had to be born, Tyrael had to fall from haven and both Baal and Azmodan had to leave hell. Of course it'd be highly unlikely that they'd stay content with their warm domains... But what if they decide to wait... 100 years? Leah is long dead, not mentioning Adria (unless she really was 1000 years old when D1 events took place). No one knows about about black soulstone- and surely no one knows where to find it. Because Leahs powers were needed to invoke Zoltans ghost, even if someone finds his head, he won't know what to do with it. Everything before point this can be dismissed. But how any plan can base onsuch incredibly unlikely set of events?
Leah- it's known that she has some sort of power in her. She's raised by Cain... Who seemingly completely ignores it. Why does she have such otherworldly power? It's not after her mother as it's totally different kind of power, it's definitely not after Aidan- he was a warrior after all. And Adria trains her, apparently knowing source of Leahs power and yet- no one is concerned. Until it's too late of course...
Summarizing- I'm deeply disappointed with D3 story. It has dozens of flaws, is incoherent and seems like it's been created by someone who barely read through lore. It's especially sad as D2 story was basically D1 story expanded. We've had almost everything in place, there was no single thing (until D3 story has been revealed) that wasn't back in D1. Granted- it wasn't too complicated story, but it had dozens of smaller plots perfectly fitting into lore. D3 seems like trying to do this, but it's tripping over its own foots- not mentioning about good connection with previous games of the series.