Don't you think she's curious about what the President's schlong looks like?

I think the game is resembling Warcraft simply because Blizzard designers are learning a lot from the game mechanics specifically behind World of Warcraft.

They're expanding the color palette most likely to assist with the pacing of the game, and that constantly shifting contrast ("first you're in a really bright desert, then you're inside a really dark pyramid") propel that sense of progress that players have as they move through the game.

It's one thing to have that gritty, dirty visual style in a dungeon instance that's supposed to last for an hour or two, it's another to have that exact same gritty visual style for the entire several hundred hours that you'll be playing the game.

One of the ways that playability is enhanced, and monotony is prevented is by having that really extreme sense of contrast, as well as the bright color palette.

Furthermore, I understand that most Diablo players don't want a color palette that looks like it was extracted from a Night Elf starting zone, but by the same token I feel like Blizzard wants to reach out to the millions of folks in the WoW contingency that might want to start playing Diablo for the first time if it looks and feels like something they are already very familiar with.