As someone that isn't very fond of classes with darker story built into their design, I was pretty disappointed when shadow priests went from having a neutral resource like mana, to using insanity as a resource. Prior to the change I saw Shadow priests as being more elemental and neutral, rather than being darker and on the edge of the void.
I know this might be a funny thread to make considering some of the abilities that spriests have had for a long time and how shadow generally is related to the void, but I don't pay much attention to the story and lore. Regardless, as someone that doesn't pay attention to the story and lore, I wasn't pleased with the change in resource and what the change suggested about the characters themselves.