FOURTH TIME'S THE CHARM?
Halo 4 possesses a duality similar to Halo 3's, with a larger story providing the backdrop for a more personal one. In Halo 3, The Flood and the Covenant had invaded Earth, and the Chief had to stop them by whatever means. But more important to fans was the rescue of Cortana, the sassy purple AI lost in space during Halo 2.
Likewise, Halo 4's most compelling story elements lie in the onset of Cortana's rampancy, a terminal condition that affects AIs who've been "alive" for too long, and the Chief's quest to get her back to Earth for treatment. In between battling the Covenant and the new "Promethean" enemies, consulting with long-dead holograms and seeking ancient artifacts, and fleeing the Sauron-like eye of the nefarious "Didact," emotional scenes between the Chief and Cortana provide the plot's real momentum.
A number of other characters are introduced, and like the returning cast (Cortana in particular) each is given new life by 343's decision to use performance capture for the first time in the series. The Chief and Cortana have never had more feelings than they do in Halo 4, and their rather taboo romance, such as it is, has never been more overt. Your reaction to that may differ, but I personally enjoyed seeing a more human side of the 9-foot soldier and his computer companion.
Unfortunately, the backdrop for that personal tale isn't nearly as compelling. Admittedly, I've yet to read the most recent Halo novels by Greg Bear—the ones focused on the ancient Forerunners—that would have no doubt lent more context to Halo 4's larger narrative. But I'm guessing most of the people about to play Halo 4 haven't read them either, and as a result the whole story will suffer from seeming rather out of context. Villains like the Prometheans and the Didact are totally out of left field, and even after lengthy cut scenes, their motivations and actions remain ambiguous and inscrutable.
Multiple play throughs will likely help clear the fog, but I also suspect that most Halo 4 players will complete the campaign once—if at all—before moving on to the multiplayer. And to them, just like for me, it's not going to make any sense. I'd argue that the Master Chief should have been left to his endless, dreamless sleep, but then Halo 4's marketing wouldn't have been nearly as compelling, would it have?