Such as armor pieces like in reach?
The Raider armor for Halo 4 is unlockable via entering codes in the “classified section” of your Career tab after finishing the first three missions in Spartan Ops. You can find the codes by Google.
A skin for the Raider armor is also now available with certain codes, but you have to complete all chapters for Spartan Ops on Legendary, finish the Legendary Campaign alone, and finish 1000 war games matches first to unlock the whole set.
At this point, I don’t know if there will be more.