Nephalem Valor is a buff that increases the player's Magic Find and Gold Find by 15% per stack and can stack up to 5 times for a total of +75%. Only level 60 players can gain the buff by killing champion monster pack or a rare monster (not Unique Monsters). The buff persists through death but it will disappear when: skills are changed, leaving the game, or after 30mins of not refreshing it. As an extra reward, the buff can guarantee additional drops and higher quality loot when a boss is defeated.
The buff is intended to alleviate several problems the developers faced with the end game: to give players a reason to stick to their builds, to discourage running one area for maximum drop efficiency and yet still make boss kills worthwhile. With the Valor system, the player is encouraged to first clear a level in search of elite monsters, eventually completing it with a boss kill.
Confirmed bosses that are affected: 
- Jondar, Skeleton King, Queen Araneae, The Warden, The Butcher
- Maghda, Zoltun Kulle, Belial, Treasure of Khan Dakab chest
- Ghom, Siegebreaker Assault Beast, Cydaea, Azmodan
In patch 1.0.3, a full five stacks of Nephalem Valor guaranteed one rare item from all champion and rare packs, and one rare instead of two from bosses. As of patch 1.0.3b, all bosses are guaranteed to drop at least 1 Rare item for players with 4 stacks of Nephalem Valor and at least 2 Rare items for players with 5 stacks of Nephalem Valor.