Personally, I'm not a fan of what the Mystic was. She was too like the Torchlight enchanter, who I totally hated. I'd rather just have my best gear be my best gear, without it needing the best random enchant to make it the best best.
Unique kill animations were never ever a planned "thing." It was a one-time special animation controlled by another dev through commands to make the Siegebreaker fight more awesome. Its purpose was that and only that. In fact, that whole fight, from the trees to the skeleton horde was unique in that it was just to make the game seem awesome to the people watching.
Needless to say, it worked.