Diamond Camo (Multiplayer):
Unlock the gold camo for every type of gun a given class can use and you'll unlock the diamond camo for that class. For instance, you'll need to unlock the gold camo for every sniper rifle if you want to obtain diamond camo for the sniper.
Jimmy Kimmel easter egg:
There's a Jimmy Kimmel easter egg in the game. Near the end, you'll see Raul Menendez sitting in a prison cell while someone on the television remarks that Menendez was invited to be on the show but had other plans.
Thor's Hammer easter egg:
You can view Thor's Hammer in the game's early Celerium mission. Once you fly down to the area where guards are patrolling, take out the guards and drop into a ravine. Look around and you'll find some brush hiding the entrance to a cave. Check inside the cave to find Thor's Hammer, though you'll have to leave it where it sits.