Tiny Details and References
The moment you wake up from your dream at the start of the game, look above Marcus's bed and around his room. He has a copy of the game you're playing.
At the very start of the game, when you meet Jace, he's trying to get a pack of mints out of a vending machine. Specifically, Mustard's Infinity Mints. It's a tiny nod to the Mustard brothers, who made the game Infinity Blade.
By the way, once Jace gives up, you can give it a try yourself with Marcus. He'll kick the machine and grab the candy that falls.
In the same room, there's a Thrashball table. This one, and any other one in the game, will play sound bites from Cole Train's glory days when you use it.
In the final cutscene of the game, when Marcus stabs the queen of the Locust, the knife he uses is the one Dom passes him much earlier. Dom gets his revenge, at long last.
Play a match on the Gridlock multiplayer map and complete it without destroying any of the ash people. It's hard, but possible. If you manage it, the song "Mad World" will play.
Also on Gridlock, if you climb up to the tower area you can look through some boards and see a fire blocked by some ash people. Shoot the ash people and you'll see a pig roast. Shoot the handle of the spit to turn it, if you want a more even application of heat to your pork.