Nephalem Valor is a new system introduced in Diablo III. It is intended among other systems to make sure that the end game is fun and exciting and also that the game is balanced so a 3-minute run of the same area won't be the most efficient way to acquire loot.
Nephalem Valor is one of the major new systems in Diablo III and it kicks in at level 60. The philosophy stated by Blizzard is as follows: Rare and Champion packs already have great loot on them. By killing a Rare or Champion pack or opening a resplendent chest, not only do you get the loot, but you'll also receive a buff granting you increased magic find and gold find. Completing an Event awards you Nephalem Valor as well. However, if you change a skill, skill rune, passive, or leave the game, the buff disappears. As an extra reward, if you kill a boss while this buff is active, you’ll receive extra loot drops from that boss.
In the PC version, Nephalem Valor can be stacked as high as 5 stacks of Nephalem Valor, each stack offering +15% Magic Find, Gold Find, and a multiplicative modifier to EXP gained. The Magic/Gold find is allowed to go past the maximum cap of 300% by however much the stacks grant. So players who have already reached the cap can still accrue up to 75% Magic/Gold find from their Nephalem Valor.
For console games, the exact amount of magic find and gold find provided by the buff is +25%. It can stack up to 3 times (to a total of +75%) and will last 30 minutes. The duration can be refreshed by killing additional elite packs, opening a resplendent chest, or completing an Event. However, the buff will not grow to more than a stack of 3 (+75% gold find, magic item find, and experience).
This system was implemented to encourage players to stick with a skill build of their choice, select an area of the game they enjoy, and sweep it for elite and champion packs on their way to a boss (the buff is not refreshed when killing a rare - violet - enemy), finishing off a run with a boss that'll be worth killing. If you wanted a shorter play session you could be done at that point, but if you have more time, the path of least resistance would ideally be to stay in the same game and make your way towards the next boss.