Just got my new issue of Game Informer in, had some nice info on Halo 4 in it. I won't talk about the campaign stuff too much, but basically it explains that they are going to be focusing more on John' (the Master Chief's) story, and how Cortana is nearing the end of an AI's expected 7 year lifespan.
Main thing that I find REALLY interesting is the direction 343 industries is going with the multiplayer. Where as previous halo games didn't really have much connection between multiplayer and campaign, in Halo 4 during multiplayer you are a Spartan IV stationed on the
UNSC Infinity. All the multiplayer maps and such are basically created in a holographic training room (think Danger Room from X-men or holodeck from Star Trek).
Now, you might ask - ok, what are we training FOR exactly? Simple answer - think Spec Ops missions ala Call of Duty. Better answer - These aren't just random missions like CoD. Each week, 343 industries will release a cinematic cut scene that explains the story of the UNSC Infinity, along with 5 co-op missions tied into this cut scene. The entire thing is named the "Spartan Op Season" and will largely work like a TV show. Each week a new cut scene and 5 missions get released over a period of months. REALLY looking forward to this game now that I know about that feature!