Playing on difficulty level hard: When wearing full diamond armour with protection IV, you will still get killed in one shot by a creeper explosion in moderate range. Horses with diamond armour, 14 hearts will also also die instantly. Other mobs don't do anywhere near the amount of damage (skeleton arrows do 1 heart, zombies do 1/2 to 1 heart of damage). Not entirely sure if this is an intentional consequence of rebalancing.