Terminator 2 Movie Easter Egg:
During 'Mission 3: Meltdown' in Story Campaign mode you will come across a Foundry facility that has lava everywhere. If you jump in the lava and stay there until you die your Marine will give the thumbs up like in the Terminator 2 movie where the T-800 Terminator dies in the lava.
Unlock New Game Plus:
Complete the Story Campagin mode once to unlock a feature that is similar to 'New Game +' where when you complete a mission you will be given the option to 'Continue'. Doing so will allow you to keep ALL your upgrades on subsequent playthroughs.
Hidden Super Turbo Turkey Puncher 3 Mini-Game:
Originally featured in DOOM 3 this mini-game can be found in the 'Advanced Research Complex' mission in the MIXON hallway that has the grey storage blocks stacked up in the corner.
Unlock Ultra-Nightmare Mode:
Althrough this feature for the Story Campaign mode is available from the start you must play through the game on any difficulty setting up to the end of 'Mission 6: Into the Fire' to unlock it.