Looks like two things thus far, the first being Master Chief Avatar Armour, which will allow you to dress up your Xbox 360 Avatar in John-117's Mark V Mjolnir Armour. The second is an exclusive Skull to the game: the Grunt Funeral Skull. It's basically the Grunt Birthday Party Skull on crack, and you can check it out via the video here or below.
Now, exploding Grunts aside, what I loved about the video above was how nice the graphics really look, and how the Assault Rifle is now a proper MA5B model and not the MA37 model featured in Halo: Reach; better overall continuity.
I also confess that I am quite interested in Halo: Combat Evolved Anniversary, but thus far I'm seeing it carry a price tag of $39.99 and I personally feel that's far too much for a simple HD remake. Should I find it for $29.99 or with a Best Buy deal of 800 Reward Zone Points ($10.00), then I'd bite at launch.
Regardless, you can read 343 Industries official announcement right here.