This game is exhibitive of an industry trend to de-emphasize the role of Heads-Up Displays: it lacks a life bar, aiming visor and ammunition readout (the ammo readout, and aiming visor, can be turned on and off at the player's will.), further adding to the challenge and encouraging the player to find alternate weapons and techniques. When you encounter the monster it is invincible. As Hud, you can only kill the parasites. You can use objects to your advantage or you can shoot them with your gun.
As The monster on the other hand, you go around an open world New York destroying buildings and fighting the military. The Monster has an arsenal of abilities. He can shed parasites, and eat people when his health is low. Everything in the enviroment is totally destructible. You can free roam, or you can play missions where you must fight the military or destroy a certain amount of buildings within the time limit.