BackFire
Blood. It's nature's lube
So this was by far the best storytelling in a Halo game. I was very impressed with that aspect of it. Storytelling has always been something I felt was lacking in the series, I just never really felt a connection to anyone in the series, and never really cared about what was happening on any meaningful level. That all changed here.
Spoiler:
Cortana's death was extremely sad and well done. Also it was great finally seeing Chief show some semblance of emotion and feel human for a change. That said, I was a bit surprised as to how the game didn't really set up the rest of this new trilogy. This could be a stand alone story with the way it was done. With the Didact dead, I wonder where they'll take the story in the next 2 games.
Overall it's definitely up there with Reach as the best Halo game as far as the campaign goes. I felt there wasn't really enough freedom as far as choosing what guns to use. I was pretty much forced to use the forerunner weapons for what felt like most of the game. They were fine weapons - fun to use and all that. But I usually prefer using the human versions and I couldn't for good portions of the game.
Also I like fighting against the covenant more than the new enemies. It feels like there's more to them and more strategy involved when fighting them.