Insane Difficulty
Complete the game on either Casual or Hardcore difficulty to unlock Insane difficulty.
Secret Gamer Pic
Complete the game on Insane difficulty to unlock a secret gamer picture. It has the Red Gear's Symbol behind Marcus Fenix.
Secret Gamer Pic #2
Unlock the "Seriously ..." achievement by getting 10,000 kills in ranked multiplayer matches to get that respective GamerPic

Achievements
Complete each achievement to get the allotted gamerscore.
Unlockable How to Unlock
The Money Shot (20) Kill 100 enemies in ranked matches with a headshot.
Always Remember Your First (10) Finish playing a versus ranked match.
Don't Hate the Player (10) Finish with the highest points in a ranked match.
Mix It Up (30) Win a ranked match in every versus game type.
Around the World (30) Win a ranked match on every versus map.
Can't Touch Me (20) Win 10 ranked matches without losing a round.
Seriously ... (50) Kill 10,000 people in versus ranked match total.
Prison Breakout (10) Completed tutorial level on any skill level.
Completed Act 1 on Casual (10) Complete Act 1 on Casual Difficulty.
Completed Act 2 on Casual (10) Complete Act 2 on Casual Difficulty.
Completed Act 3 on Casual (10) Complete Act 3 on Casual Difficulty.
Completed Act 4 on Casual (10) Complete Act 4 on Casual Difficulty.
Completed Act 5 on Casual (10) Complete Act 5 on Casual Difficulty.
Mercenary (10) Complete all acts on Casual Difficulty.
Completed Act 1 on Hardcore (20) Complete Act 1 on Hardcore Difficulty.
Completed Act 2 on Hardcore (20) Complete Act 2 on Hardcore Difficulty.
Completed Act 3 on Hardcore (20) Complete Act 3 on Hardcore Difficulty.
Completed Act 4 on Hardcore (20) Complete Act 4 on Hardcore Difficulty.
Completed Act 5 on Hardcore (20) Complete Act 5 on Hardcore Difficulty.
Soldier (20) Complete all acts on Hardcore Difficulty.
Completed Act 1 on Insane (30) Complete Act 1 on Insane Difficulty.
Completed Act 2 on Insane (30) Complete Act 2 on Insane Difficulty.
Completed Act 3 on Insane (30) Complete Act 3 on Insane Difficulty.
Completed Act 4 on Insane (30) Complete Act 4 on Insane Difficulty.
Completed Act 5 on Insane (30) Complete Act 5 on Insane Difficulty.
Commando (30) Complete all acts on Insane Difficulty.
Time to Remember (10) Recover one-third of the COG tags (on any difficulty).
Honor-Bound (20) Recover two-thirds of the COG tags (on any difficulty).
For the Fallen (30) Recover all of the COG tags (on any difficulty).
My Love for You Is Like a Truck (30) Defeat a Berserker on Hardcore Difficulty.
Broken Fingers (30) Defeat a Corpser on Hardcore Difficulty.
A Dish Best Served Cold (30) Defeat General RAAM on Hardcore Difficulty.
Zen and the Art of Reloading (10) Perform 25 perfect active reloads (on any difficulty).
Zen and the Art Part 2 (20) Perform 5 perfect active reloads in a row (on any difficulty).
Clusterluck (20) Kill 3 enemies at once 10 different times (on any difficulty).
A Series of Tubes (20) Host and complete 50 ranked matches.
Dom-Curious (10) Complete 1 co-op chapter as Dominic Santiago on any difficulty.
Domination (20) Complete 10 different co-op chapters as Dominic Santiago on any difficulty.
I Can't Quit You Dom (30) Complete all acts in co-op as Dominic Santiago on any difficulty.
Don't You Die On Me (10) Revive 100 teammates in ranked matches.
Fall Down Go Boom (20) Kill 100 enemies in ranked matches with the Boomshot.
Pistolero (20) Kill 100 enemies in ranked matches with a pistol.
The Nuge (20) Kill 100 enemies in ranked matches with the Torque Bow.
I Spy with My Little Eye (20) Kill 100 enemies in ranked matches with the Longshot.
Don't Hurt 'Em (20) Kill 100 enemies in ranked matches with the Hammer of Dawn.
It's a Massacre (10) Kill 100 enemies in ranked matches with the Chainsaw.
Curb Appeal (20) Kill 100 enemies in ranked matches with the curb stomp.
Capital Punishment (20) Kill 100 enemies in ranked matches with an execution.
Is It a Spider (30) Kill 100 enemies in ranked matches with grenade tag.