Even though pretty much everyone else expected it, I was totally surprised to see this. But man did I flip a shit once I saw Chief's hand, as anyone in the Shoutbox in the time surely noticed. XD Man, my mind's been racing since the reveal as to what the plot will be this time around.
So far it looks like Chief will be searching for Cortana in some way, as evidenced by the AI chip Chief is carrying with him.
His cloak and the state of disrepair his armor is in (or even the fact that he's still using his old armor to begin with) seems to indicate he's no longer working with the UNSC, and has possibly gone rogue or even just deserted. Given that most of humanity didn't appear to know Chief was alive based off of Halsey's reaction in Spartan Ops, it seems whatever rift that occurs between Chief and the UNSC must have happened shortly after the end of Halo 4.
Lastly, the predominant theory as to the large Forerunner being is that it's an ancient War Sphinx, an extremely powerful Forerunner war vehicle/mech.
Oh, and there seems to be something odd going on with the game's name... Mostly in that it doesn't have one. Oddly, 343i isn't calling it Halo 5, even unofficially, and has simply stated the name will be revealed later. Seems odd that they would change the naming conventions at the second game in a new trilogy, rather than the first game, but whatever haha.