Though I'm not terribly far through it, I would say that it does have a more diverse colour palette but that is certainly to it's credit as it's definitely not like everything is in Rainbow-Technicolour or a Neon-Rave.
It's a Blizzard game, so you can expect their usual manifestation of perfection but I think what trips people is the fact that the last Diablo came out in 2000; nearly 12 years ago and a great many strides have been made in the industry since that beast was 1st unleashed.
Atmosphere-wise, I think Diablo III achieves a superior sense of it as it's able to utilise technologies that D2 couldn't have hoped to employ; things like proper shadows and lighting, shaders, rolling fog, destructible environments, wind & rain, etc.
Baring in mind, the Beta only covers the 1st Chapter of the 1st Act and hence what you're presented with is but a fine sample leaving you wanting the main course feverishly.
Rest assured, if anyone has ever played the previous games and enjoyed them; You're definitely going to want to own Diablo III.![]()