What I´m really talking about is Tristram from Diablo I, specifically. Take a look at the collors of the grass, the rocks, everything... It´s almost like a painting. It had a very strong mood about it, that Diablo 3 is certainly missing, because the collors are more generic, more common.
The mixing of the music and the colors, and the quiet environment, the very few people... That´s what makes Tristram... Tristram! Sadly we no longer see that, but hey, maybe they will create something even better in Diablo 3.
Sorry, but it´s not about being dark, and definitely not just about colors. It envolves the dynamics, the state of things, the disposition of the elements on the set, the set itself... everything. I think we could feel something similar in Diablo 2, specifically in act 1 and act 3, but not at the same level. And even in 3D they could still capture the same feeling as they did in Diablo 1. The graphics are very diferent, it´s pretty noticeable. And about reality, it´s really not supposed to feel realistic at all, I think. It´s a fantasy game.
Take a good look at Tristram (Diablo 1). Walk around the town. Go to the corners, where you don´t even have NPC. What is great about it is that they used a very cohese pallete color, so that you can feel a very strong mood about everything.