I don't like seeing Diablo until the very end. And when I do, I want the room to be dark or grim. That's what made Diablo so terrifying or ultra-badass. In 1 you go levels after levels to see the world get much more Hellish 'til you finally unlock his room. Even then it's a fucking bitch to see unless you loved to dump light radius on your character.
The second has this eerie path of feeling we're going straight to his door step and fighting him. The scene changes from the lava vision to a dark red one that gives the whole fight a different feeling of uberness. Granted, farming will phase the effect out but hot damn was it cool the first couple times I witnessed it.
We don't have that in D3. We have some cool fights but Diablo isn't terrifying. You can't the Lord of Terror not be scary at some level. I enjoyed the whole Leah ordeal, I just wish they made the she-Diablor or whatever it turned into something spooky and not a cartoonish villain that needed to shout everything they were doing.