It's been set up that Master Chief has defied orders and has gone rogue, taking Blue Team with him. Fireteam Osiris has been tasked with tracking them down. I don't think it's clear if he has orders to capture or kill. It's also not clear exactly what the Master Chief is up to, but it's pretty apparent that he hasn't actually turned traitor, he's just doing what he needs to without dealing with UNSC stupidity and politics. Also, I think it's pretty widely accepted (assumed?) that he's still looking for a way to bring Cortana back.

It also appears that ONI has been spreading lies that Chief is dead, as a rogue Spartan-II would probably be bad publicity, as there are already some details about the origins of the Spartan-IIs being leaked to the public.

I haven't kept up with a lot of the newer stuff, either, but I think most of that is an accurate, very general assessment of the situation.

Buck is now a Spartan, so that's obviously a big reason to be excited (although the novel about him becoming a Spartan isn't very good). I may be completely wrong, but it also seems to me that 343i may be setting things up for Locke to be taking the Chief's place as the main protagonists at some point in the future, but that's just an assumption on my part.