The biggest complaints I have heard are 1) it was too easy, and 2) the graphics are unimpressive.
In response to 1, It was a beta... They only had "normal mode" available, which is the easiest mode. Normal, Nightmare, Hell, and Inferno are the difficulties. As expected, the greater difficulties reap greater rewards. I don't think anyone is going to be complaining about it being too easy when they try Inferno, yet those who wish to only play very casually won't find themselves overwhelmed if they want to play on normal.
2 is personal preference. I'm not a stickler for graphics, so I don't care. I understand that Blizzard wants to make the game playable on older machines in order to appeal to a wider demographic.
I loved the gameplay. I'm excited. I have purposely jacked up my sleep schedule so that I'm sleeping in the evening and waking up around ~2am, so that on Tuesday, I can wake up and pull a 48-hour energy-drink-infused gaming binge. One of the dudes I roll with is hell-bent on world first level 60, and also very interested on trying to make money in the real money auction house by acquiring high level items first. I'm interested to see how much it's possible to make.