Kotaku writes that Blizzard had planned Diablo III as a battle through hell, as expected, but also heaven. Some of the screenshots show areas called "Heaven" and the "Angelic Lands." In these areas, the player faces snakes, snake-men, and gritty winged creatures.
The remaining screenshots are of an area called "Keep." No enemies are shown in the Keep, but we do get a solid look at its architecture. Overall, the art style looks very similar to Diablo II, but using 3D graphics.
Today, Diablo III looks much different (see below), and that's to be expected. Blizzard hasn't been sitting around on its thumbs for the past five-and-a-half years. Not only does it look different, but the current version of the game has a totally new style. Maybe Blizzard wanted something that stood out more from the previous entries in the series?
Source: The Escapist
Screens in the link. Not trying to take away from the current build which looks great; but the old one is very slick and in my opinion would still be very playable today.