Well into Act 3 now and deciding it is a damn good game.
Notably, there has been a lot of effort spent on making the world feel a bit more real and depth given to all the characters. All of your hirelings and the major characters in the story arc have history and their own problems to deal with, and your interactions with them will vary depending on your character class.
Previous Diablos had extensive world history but no real storyline beyond killing everything that moves. While that still aplies here if you want to power through the game, you can also take the time to soak in the ambiance and atmosphere of the various areas.
Many of the side charcters also have spoken dialogue that either triggers when you get near them or can be activated if you choose to talk to them. None of this impacts the game in any way, it is just there to add depth - and I love it!
I like to walk around the town areas just to listen in on these conversations, which make the townsfolk feel like real people, and add meaning to your slaughter of the demon hordes.
The gameplay is very smooth and the skill balance seems good, although there will no doubt be some tweaking over time.
My one issue remains the WOWification of the game though. There are no longer any decisions that are irreversible after character creations.
Class, sex, name, hardcore or not. These are the only choices that you make that have long term results and they are all made before you start play. After that you can redo any skill choices any time you like, with the only restriction being a 10 second cooldown to stop you re-speccing mid battle - although in a group game you would even be able to do it that way if you could lose agro for a while.
This means every level 35 barbarian is exactly the same as every other level 35 barbarian, stats identical, skills identical, only items creating any variance - and of course items can be traded.
Overall though, I have to say Blizzard has done this one very well, and I expect to spend many more hours playing.
Never underestimate the power of human stupidity!