Most of Halo 4 takes place on Requiem, a planet that was built by the ancient Forerunners, and it gives the development team at 343 Industries the chance to drop the Flood entirely from the game, in order to give us an all-new enemy race, the Prometheans.
The problem with them is that they are simply not different enough from the Covenant when it comes to battlefield interactions conducted via kinetic energy, at least regarding their weapons, which are always a focal point in a Halo game.
All players have their favorite tools of destruction from this game universe (mine is, for simple weirdness, the Needler) and one of the defining aspects of the Halo series has always been the need to scavenge weaponry on the battlefield in order to progress (check out the new Famine Skull for an interesting ammo-based experience).
But the weapons that the Prometheans drop when you finally begin to kill them in numbers are just upgraded versions of the ones that the United States Space Command and the Covenant carry, although with some unique animations and some nice lighting effects.
This is a big letdown and the fact that the Prometheans actually behave in different ways on the battlefield has limited compensation value.
The Knights are fearsome enemies because they are continuously supported by their drones and you need different weapons in order to take the two kinds of enemies out.
Their pet-like companions are also a challenge because they are very quick and tend to draw your fire while the Knights come close and deliver the heavy damage.
I have fun while fighting the new enemies and I love the way they engage the Covenant when both are around, but the entire experience would have been even better if 343 Industries had designed some nice new toys to play around with on the battlefield.