According to what I saw in the preview, the gameplay is a 5 of 5, a perfect score in every sense; but in terms of originality it doesn’t offer anything new, as it is similar to both of the predecessors in this series.
The story of Diablo 3 is set 20 years after Diablo 2, after the unnamed heroes of Diablo 2 saved the world. But the price was that the heroes who survived the battle lost their sanity due the fight between the Burning Hell's armies, and it is up to a new generation of heroes to protect the sanctity of Sanctuary. There is no more information about the story of the game beyond the basic premise.
The graphics and sounds both are a five, they really interesting so far. You feel the action through the music, and the graphics are improved from the previous installations. The playability is still not known because the game hasn’t been release to the public yet and only a few people played the game during the convention in France last year, but according to some reports there aren’t any major changes to the control system and the players won’t feel any major changes at all.
As I said, the gameplay so far is completely perfect as a game of this type can get, and the fans of the Diablo franchise can only hope that final product dosen’t diverge at all from the demo, as we have seen many games that had a perfect demo but not a good Gold development. So, since there is no game yet to play to review, and we can only go off of the demo for now, this game is getting a 5 out of 5.