You might actually have a good handle on things if you weren't so biased in your clear hatred of this game's story. I, a high school student with a love for writing and story, can see the faulty logic in your posts.
For one thing, stop using reductio ad absurdum to prove your points. It really kills the buzz of the story to dumb down and make things look stupid just to make yourself look better. If you can show me one story where things could not have been made easier for the characters by the writers, I will pick up a bible and start preaching for a living. EVERY SINGLE STORY has moments in it that are filler, or are cliche (because nothing is original anymore), or are exorbitantly long or don't really contribute anything. You seem like the kind of person who would say "In Lord of the Rings, why didn't they just fly the eagles to Mordor and throw the ring in Mount Doom?!", or "In Star Wars, why didn't Darth Vader kill the Emperor with his lightsaber so he didn't die himself?!". The writers use filler and long plot lines because the game is not about the beginning and the end, it's about the journey in the middle. Notice how that virtually no part of any movie or book dwells on what happens before or after the quest? You don't see Tolkien writing a whole other book about how perfect life was after Sauron's spirit was vanquished, do you?
You have a few good points, though, I'll give you that. For one thing, Belial and Azmodan were much too unlike their own characters. Belial should have been much more slippery and tricky to kill (he is the Lord of Lies), and I feel like Azmodan, as a brilliant commander and demon strategist, shouldn't have followed the mistakes of every major villain in the history of stories and told us every one of his plans and made us come to his lair in hell. It would have been much more compelling if Azmodan had taken a larger role in the battle of Bastion's Keep, such as making us delve into his lair in hell, only to find that he himself had launched a personal attack on the Keep, so we have to run back and kill him there. Also, Belial wasn't tricky enough. He's the lord of lies, shouldn't he have agents everywhere that look like our friends, or use that clever mind of his to elude capture and trick us into killing allies?
Your points about Tyrael, though, are, quite bluntly, stupid and uninformed. He's acting like a kid who threw a tempter tantrum?! Did you not watch the cinematic that you so loved? He's the archangel of justice, and the most righteous angel on the Angiris Council, so when Imperius and the others refused to help the innocent, which is the right thing to do, wouldn't it make sense that Tyrael would refuse to stand alongside those who would let an entire race die? He gave up his power because he would rather be a mortal then be an angel who wouldn't do anything while the demons slaughtered the humans. He's so willing to do what is right, that he gives up his power and his wings, the most coveted mark of an angel. Also, how was he supposed to know that his fall would raise the dead and King Leoric? Has an angel ever fallen in the Diablo series before? This is not ignorance presently, it is only ignorance in HINDSIGHT, which is most certainly not Tyrael's fault. It's like half the drama films and shows in existence where the character does something that we know about, because we're omnipotent, and they don't, and then calling them stupid for not knowing something that we do.
Your points about Adria, as well, are irritating. This woman is Leah's mother, not some stranger off the streets. She was a respected witch in Tristram and friend to Deckard Cain, doesn't that bring some credibility?! Besides, it's not like she was completely inconspicuous, there's a moment in Act II, before we release her, that our character warns Leah about her mother that things are not always as they seem. Plus, once we capture Azmodan in the Black Soulstone, she specifically says that she cannot shatter the stone right away, she needs to prepare rituals and incantations, and even if that wasn't true, again, by now we trust her. Trusting Adria isn't exactly like trusting a bandit who just tried to kill you. She's never done anything bad right up until we betray her. Again, it's only ignorant in HINDSIGHT. You keep making the conclusion that because we know something as an omnipotent observer, and the characters don't, that the characters are stupid.
For a grad student in literature, you are remarkably uniformed and hostile in your opinions. I saw all of two good points in your posts, while the rest was just trolling. You keep talking about how the story could have been made easier for the Nephalem and for Leah and Tyrael. Well, good for you. The next time a fallen star hits earth and a huge army of demons pours from Mount Vesuvius, I'll give you a call. It's not about what is easy, it's about what is epic. THAT is what makes a great story.