It's not the lack of gothic art or too many colors.
It's the difference between the awesome concept art and the godawful low-poly ingame model. There is no grit, as in contrast and actual detail. Everything seems soft, much like WoW. Diablo 2 may have been colourful and varied, but it tried to be pretty photorealistic or at least it had an insane amount of detail and grit. This... this looks cartoony, which might fit in Warcraft and Starcraft, but not Diablo.
Also the whole dual-wielding crossbows is dumb. Maybe if they had been fully automatic crossbows ala Van Helsing it wouldn't have looked as ridiculous, but that trailer had me laughing at the bolts magically appearing from the tip of the crossbow (heck, they don't even look like bolts, just little white lasers) as if she was wielding a gun.