Diablo – What events grant a stack of Nephalem Valor

The entire Nephalem Valor concept has been removed since Patch 2.0.1 (live on Feb. 25, 2014). Along with this change, two new concepts have been added to the game (Official Patch Notes):

Pools of Reflection have been added to the game

• Pools of Reflection provide a 25% bonus to experience
• This bonus persists until you reach a set amount of bonus experience (equal to approximately 10% of the current amount of Paragon experience needed to reach the next Paragon level) or your character dies
• Gathering additional Pools of Reflection bonuses increases the amount of total experience bonus a character can receive until character death, stacking up to 10 times
• This experience bonus persists on logout

Nephalem Glory has been added to the game

• Occasionally during battle, monsters will drop Power Globes when damaged or killed
• Power Globes grant the Nephalem Glory buff, which causes players to deal additional damage in the form of golden lightning bolts and increases their movement speed for a short time
• This buff can stack up to three times

Most enemy spawns are completely random.

The only types of special enemies that spawn deterministic are event bosses (events and sidequests with a "mini-boss" will always have one, although not necessarily always the same) and main-story quest-related monsters.

But since events themselves are randomized, in and of themselves they are not worth focusing on.

It is the case that some areas seem to have a higher density of elites and other high end mobs. Their spawns within an area are not deterministic per se, but if you clear the zone you will tend to find more in some areas than others. One particular area of note that fits this mold is the Vault of the Assassin in Act II.