It is vastly different
There are several things i will list
The campaign features new appearences of weaponry, enemies, and plot structure.
Spartan op's is a new feature with updated missions weekly
Multiplayer includes customization armor, armor abilities, and new maps, gameplay and design.
Overall whats been revealed is nice.
For more info search my name, Protomario on youtube, I have videos up about it and I am a partner.