One of the problems I had with the story in Diablo 3 was that none of the characters had any substance. Coming from Diablo 2, I was very keen to see who we would be fighting and why and how. However, I get given a one line dialogue "Oh yeah there's a couple of other lesser demons that we never really mentioned before, now they're up to mischief and you have to kill them". I haven't read the books, obviously, so hearing this was a pretty huge anti climax. Perhaps some better introduction? Oh Belial is attacking some city I've never heard of? Oh okay... Maybe a cut-scene near the beginning showing what was going on with these demons? The only time you saw them was when you were about to kill them. I had no vested interest in killing them other than that I'm supposed to. Boring.
This isnt even to mention all the other little mini-bosses who you never hear about until you are actually killing them. This is not how to create character.
Moreso, whenever a piece of the plot was 'resolved' or often told, the appropriate person would just appear next to you. "Oh hey Adria, you just appeared next to me through that portal you can happen to make anywhere you like"... "Oh hey spider lady, you just happen to appear next to me on your web out of nowhere... who are you again?" ... "Oh hey angel people, thanks for randomly appearing after everything is over to congratulate me, you know these teleporting abilities would have come in handy earlier".
Anyway, what I'm trying to say is that the storytelling in this game is quite amateur IMO. Not to mention the little pages you pick up here and there (which are often fine) until you find "Azmodans Orders part 1" on a demons body. Really? So demons from the buring depths of hell receive their orders on paper from their demon lord, in English (or can we understand Demonic?), telling them what to do. "Dear Salnexaam, I Azmodan your supreme ruler would appreciate it if you would smash a lot of stuff, and kill people if you see them in the keep, oh and watch out for that hero guy he seems troublesome. P.S. If you happen to kill him I'll buy you drinks tonight".
I know, I know, they're just trying to tell us whats up, and its just a game, but seriously things like that just feel so horribly childish to me.
Finally, the overwhelming OBVIOUSNESS of the story is shocking. From the start of the game everything is pointing at Leah being Diablo... "Leah has this crazy power that is getting more powerful, we won't explain what it is or why she has it", "Check out the DEMONIC LOOKING SPELL POWER that happens every time she uses her 'power'..." or my favourite was Kuul saying "Oh if only your friends knew what you really are". Seriously, its as bad as reading spoilers.
Honestly, I could go on for quite a while about all the things I thought were very poorly done, but I realise by now I'm probably ranting.
Anyway, I enjoyed the game, but was quite underwhelmed at a story that was 12 years? in the making.
EDIT: Oh oh oh, I just HAVE to mention this too. I fought Diablo on the 'arch' or whatever. I was on solid ground. I was not near a balcony or edge. He died, his body was still on the ground. Skip to cut scene, and hes falling through the air, and conveniently there goes his soulstone falling away. No, it should have been right there on the ground. This is shamelessly setting up for a sequel and it is bad.