I think that many of us gave up on a sequel to Diablo 2 many years ago. Blizzard was such invested in World of Warcraft and didn't really produce anything else, until they broke the silence and came out with Starcraft 2. That is when I got my hopes back for a sequel to Diablo 2.
We have been looking for a spiritual sequel, and there has been many. I'm thinking about Torchlight, Dungeon Hunter and actually World of Warcraft. But they have been lacking. Lacking the universe of Sancturary, the steel guitars and the matte black of Diablo.
Still I think that Blizzard has learned a lot from World of Warcraft and brought in to Diablo 3. They've learned that the casual audience is where the money is, and that is why Diablo 3 is perceived as a much easier game than Diablo 2 was. I'm not sure that is the case, but the entry difficulty is lower.
When you gained a level in Diablo 2 you got points to invest in Strength, Dexterity, Vitality and Energy. You had to find a good balance that would fit your character. You also got skill points to put in the skill tree. Once spent you couldn't go back. If you made a mistake you were doomed with it.
While playing you would notice that you hadn't put enough points into vitality and it would take a couple of levels to fix it. Those could be hours upon hours of pain, grinding lower level mobs because you can't advance without those points.
This has been fixed in Diablo 3. Everything is free and automated.
As you cannot do anything wrong in Diablo 3, you have no investment in your character that demands knowledge or skill. All you need to do is run around and do brainless bashing. There are no choices that can't be undone, and you need not do any research on character builds.
I'm not saying it is a bad thing. I'm just saying that Diablo 3 has made Diablo casual.