Just finished the campaign. I thought that Halo 4, being the start of a new trilogy, would be a nice jumping on point to really get into the universe (I've only been a casual fan, despite being into sci-fi and shooters in general). Unfortunately, I was dead wrong. I didn't understand who the bad guy was or his motivations for wanting to destroy humanity. There was nothing to really flesh out his backstory. A brief video summary of what happened in Halo 1 - 3 would have been great. Or even a video explaining what the Forerunners are or what the hell a Composer is (I still don't know).
So I just kind of went through the motions, focusing more on the game play experience rather than the story. It was fun, better than 99% of other shooter campaigns. But I just wish 343 made it easier for casual fans to understand what the hell is going on. There was no emotional investment on my part.
Multiplayer is what really shines. Halo 3 felt so ancient when it came out. Chief couldn't run, no classes to choose from, no leveling, slow pace of play, etc. It was dreadful. Reach tried to introducethese modern innovations but they just couldn't build a cohesive package. This new multiplayer team has really hit a home run. Everything is just fantastic.