Again... this is an ARPG. The story automatically takes a back seat. It's just the nature of an ARGP! Were the story central to the Diablo experience — were you forced to experience the narrative more intimately — I'd be singing a different tune. At best, though, it's only just as important as the item variability and monster-bashing and gear-hoarding mechanics (and I'd say that's a huge over-inflation of the story's role).
I've read the manuals. They're more extensive than manuals for other ARPGs, yes — but so what? My issue is whether the lore is good enough to get bent out of shape if Blizzard should chop it up to make D3. It's fine for what it is... which is the rather generic fluff used to pad out the hacking and the slashing. If you truly believe it's even remotely on par with, say, the writing of Lewis or Tolkien, or any of the game settings ever produced by FASA or Steve Jackson Games or Games Workshop... then I can't discuss anything with you.
It's all fine. And I'm just beating a dead horse here... we'll never agree. I think the lore is fine as a backdrop, and it's interesting enough to hold my attention through a game... but even a casual examination indicates that it's just thrown together as a... how do I analogize this... as a rather crude wire frame, which is only there to hold up the heavy clay of the game's important elements. I cannot fathom any situation where Blizzard could retcon info from the novels and I'd be mad about it. It's just not that good. If they do end up retconning stuff, they'll just replace it with something of the same generic quality. Oh noes, Leoric has been replaced with... another vengeful animated skeleton king. For shame, Blizzard! He was so unique!
Well, I see what you mean Jambe, and in terms of motivation, you are right. It started out as a VERY fleshed out ARPG, which is more fleshed out than many games from most genres.
It's not primarily until it became a big IP they have put more energy into it, and fleshed it out a lot more, done away with some details that don't fit very well with the world or other aspects, making the story more interesting over all.
Sure, it's not the be-all end-all masterpiece, but a lot of it is quite interesting good, and in some cases even innovative (or at least combining elements in a new way).