Taken from the steam page:
Pre-order and receive the Limited Edition for Free!
Ranger Mode - Experience complete immersion. No HUD. Less bullets. Brutal Combat. Hardcore.
Modified Russian Rifle - A compact but deadly Russian assault rifle, perfect for claustrophobic combat.
Extra Military Grade Bullets - Coveted for their ability to kill, these bullets are also the currency of the Metro. Will you use them in battle or purchase precious resources? The choice is yours.
Metro Last Light Digital Comic - Created by Dmitry Glukhovsky
I don't mind most pre-order stuff in other games since it's usually cosmetic bullshit or extra weapons, but to remove a difficulty option and make it a pre-order incentive is kind of galling to be honest.