This feature will become available when you complete the game once. In New Game+ mode you begin a new playthrough with your previously earned inventory and resources. The following is a list of things that can be unlocked in New Game+ mode.
Unlock +3 Clip Circuit Set: Beat the game on casual difficulty.
Unlock +3 Damage Circuit Set: Beat the game on impossible difficulty.
Unlock +3 Reload Circuit Set: Beat the game on normal difficulty.
Unlock +3 Speed Circuit Set: Beat the game on hard difficulty.
Unlock Builder's Circuit Set: Collect ALL 12 Blueprints.
Unlock Comms Circuit Set: Collect ALL 36 Audio Logs.
Unlock Crafter's Circuit Set & Deep Dig Suit: Collect ALL 73 Weapon parts.
Unlock Devil Horns: Beat Classic mode in New Game+.
Unlock Earthgov, Unitology, S.C.A.F., and Alien Circuit Sets: Collect ALL of the specific set of artifacts.
Unlock Engineering Suit: Beat the game on any difficulty setting.
Unlock Flight Suit: Collect ALL 40 artifacts.
Unlock Hostile Enviornment Suit: Finish ALL 10 optional missions in Co-Op mode.
Unlock Mega Resources Deposit: Beat Pure Survival mode in New Game+.
Unlock Mk-II Overclock Parts Set: Beat Pure Survival mode in New Game+.
Unlock Mk-II Overclocked Attachment Set: Collect ALL 61 Circuits.
Unlock MK-II Overclocked Frame Set: Complete ALL 10 optional missions.
Unlock MK-II Overclocked Module Set: Collect ALL 71 logs.
Unlock Mk-II Overclocked Tip Set & Flight Suit: Collect ALL 40 artifacts.
Unlock Research Circuit Set: Collect ALL 35 Text Logs.
Unlock Retro Mode: Beat Hardcore mode in New Game+.
Unlock Classic Conflict Mode and ALL Characters in Multiplayer MAdded 12 Feb 2013, ID #5171
This difficulty setting will become available when you complete the game. In Classic mode you can only use classic weapons through blueprints, there is no co-op mode, aiming is in the original style, and the difficulty setting is locked to 'Hard'.
Unlock Pure Survival ModeAdded 12 Feb 2013, ID #5169
This setting will become available when you complete the game. In Pure Survival mode enemies will not drop health or ammunition, ALL items must be crafted at the bench, and the difficulty setting is locked to 'Hard'.