My critique would be presentation, not lore. Diablo 1 had the best presentation in my opinion. You see a body on the ground and he tells an ominous story about a demon called the Butcher. Suddenly, I have an eerie feeling knowing that I'm about to encounter something horrible. Or with fighting Diablo. Cain sets you up and tells you about him, you go down to the last level, the load screen shows a creepy silhouette of a demonic figure, and once the level loads, you go in knowing full well that the big man himself is just waiting for you to find him. In D3, you're going step by step along the missions and then All of a sudden, "fight the butcher!" Ok, random.
The first Diablo was by far the darkest and most eerie. Maybe even most graphic. I usually had to play with the lights on, especially In the catacombs. D2 wasn't dark hardly at all and D3 was a step in the right direction but still didn't deliver on that front. The Diablo series would be doing the right thing to bring back the D1 feelings of creepy, ominous, eerie gameplay.
Overall I give the lore of D3 a 9.5/10 and D3 as a whole 9/10