It's lack the seriousness of the other Ninja Gaiden Game. That's already expected if you watched the trailer. Annoying at times and lacks an opportunity for backtracking. Ninja Gaiden Z was made for fun, style and bloodlust, not exactly for an engaging story. You've got to appreciate the art style (like a comic book), a left field approach (unexpected) to the visual style of the previous Ninja Gaiden games. I would not go so far as to expect NGZ a continuation of the former series. It's a side-story, introducing a new character, hit or miss, vulgar humor.
Those who play the game,
There is nothing here if you are going to over-analyze and spend most of your time whining and moaning. The game is still a lot of fun, stylish challenging... You will die a lot while killing the undead." Because that's what the game is about. That and trying to kill Ryu Hayabusa.
So enjoy. Yes it's more annoying than funny, but hell, it's fun. It's all fun isn't it?