I think the bad writing in D3 is a weird consequence of WoW's success. Either because they hoped to attract part of WoW's audience, or for the simple fact that a lot of Blizzard's staff at the time had also worked on WoW and were more familiar with that style, both Starcraft II and Diablo III suffered from a cartoony art direction and a story that didn't take itself very seriously compared to their predecessors.
In Diablo II, Diablo greets you with "Not even death can save you from me", one of the most iconic lines in all of gaming, creepy and to the point.
In Diablo III, Diablo holds an entire monologue explaining the boss mechanics, something like, "Now you're trapped in my realm, you can only escape by defeating the shadows, but none have ever managed to do that har har har". You can't get emotionally invested in a world with writing like that.
In Diablo II, Diablo greets you with "Not even death can save you from me", one of the most iconic lines in all of gaming, creepy and to the point.
In Diablo III, Diablo holds an entire monologue explaining the boss mechanics, something like, "Now you're trapped in my realm, you can only escape by defeating the shadows, but none have ever managed to do that har har har". You can't get emotionally invested in a world with writing like that.