For example, in D2 the skills you chose were a crucial component of how well you did later in the game. You could create a build with relatively crap equipment who could do well later in the game based on their skills. But in D3 skills don't matter and everything is based on equipment - and you can buy equipment for real money and give Blizzard a cut.
Also, it doesn't seem big enough. I got my first character through normal in something like 3 days. And then I'm repeating the same old thing. It was 11 years between the release of D2's expansion and Diablo 3. I know they have other games, but they could have spent a little more time making the world bigger. My guess is they start selling service packs. They know people will buy them, so why not?
I'm enjoying it, and I'll probably play one character up to 60 and try out the others, but I just can't see myself playing it like I did D2.