OK, I'm back in. I just played through the level where you are first introduced to the new enemy type: the Flood. The Flood appear to be mutated humans and parasites who are much less intelligent than the aliens from the Covenant, but at the same time, twice as strong and fast.
The level where you're introduced to them is absolutely fantastic. It changed the pace absolutely perfectly. The first half of the level is designed to terrify the player. All you see is the carcasses of Covenant scrubs, as well as Earth marines, scattered around the indoor compound. A droning musical score sets the tone of absolute dread as you make your way through the "too quiet" level. Finally, in the deepest section of the building, you meet the Flood and are forced to fight your way back out.
The pacing in this level was as good as it gets. It wasn't too hard (didn't die once!) there was a lot of plot exposition, you get a couple new weapons, and you're introduced to a new enemy.
So just as I was about to give up on this whole game out of boredom and frustration, the designers almost telepathically answered me with this level. But guess what the next level is?
The Library.
Bring it on, yo. I'm ready.