Unlockables - Complete each achievement to get the allotted gamerscore.

Armored Attack (6) Get in the tank with two other players.
Brasso (10) Achieve the 12X score multiplier.
Combined Assault (40) Complete the game with at least one other player.
Freedom (25) Free 25 POWs.
Hero (10) Achieve the 24X score multiplier.
Humanitarian (10) Rescue all of the POWs in the game.
Madman (30) Attain a 24X score multiplier on every level.
Mercenary (15) Complete the game on Suicide Mission difficulty.
One Man Army (7) Kill 200 enemies.
Reckless Driver (20) Kill 40 enemies while in a vehicle.
Sleuth (20) Find all of the Secret Areas in the game.
Veteran (7) Complete the game on Black Ops difficulty.

Unlock Mission Select - When using the Mission Select to start on a different mission, you will be unable to earn the Humanitarian, Madman, Mercenary, Sleuth, Veteran, and Combined Assault Achievements.

Mission Select Complete the game on any difficulty