I myself agree with both sides. I can definitely see the WoW influence in the close-up of the barbarian. Lo-res textures and oversized armor just doesn't fit in Diablo in my opinion. The pastels of the outdoor area also seem odd. Having said that, the brightness doesn't bother me either. If you think back to Diablo 2 there were quite a few bright colored areas (though not pastels). Almost all of the outdoor areas in Act 3 were bright, and Act 1 had it's bright areas as well. However, once you went down into a dungeon or cave it got darker and murky. They contrasted the two to really bring out the juxtaposition of the dark areas. And really the enemies are what brought the fear-factor for me, not the environments. Knowing I could get my butt kicked around the next corner kept me much more on edge than some dark pixels on the ground texture.
In the end I would sign the petition just because I'm not 100% behind the current direction. I would buy and enjoy the game either way, but I wouldn't mind seeing a change. My main fear is that the petition has been tainted by all of the people spouting "color is so GAAAAY" and "I don't want WoW 2". This is not constructive criticism, and does not show any thought or reasoning. Those are the people who I would WANT to annoy if I were making a game, just to spite them.