When you have stacks of Nephalem Valor bosses drop significantly more loot (Source). What sort of monsters drop extra loot (not just more magical loot from the magic find, but an increased number of items)?
I've noticed the Warden and the Butcher dropping an increased number of items with Nephalem Valor but I haven't seen blue or yellow mobs dropping more.
It is safe to assume that the Act bosses count as bosses, but what is less clear is if free range bosses count. The Act bosses are:
Act I
Skeleton king. Araneae. The Warden. Butcher.
Act II
Magda. Zoltun Kulle. Belial.
Act III
Ghom. Siegebreaker Assault Beast. Cydaea. Azmodan.
Act IV
Rakanoth. Diablo.
Free range bosses are also described at that link, but it will take research by someone at 60 to answer whether they count (and I am not yet 60). I would note that champions and uniques should not be expected to count, as they are specifically mention differently in the Blizzard post.
The complete list of bosses with a 'bonus pool' for Nephalem Valor is as follows:
You'll note that, with the exception of the Warden, the main thing these all have in common is that they have a dedicated 'boss chamber' in which the fight occurs, which you cannot leave once the fight begins, which makes them easy to identify.
Everything counts for Nephalem Valor drops. See the original Blizzard announcement here.
The primary mechanism of Nephalem Valor is that it increases magic find and gold find by 25% both. So every loot drop including blue and gold mobs, but also even white mobs and chests are affected by Nephalem Valor.
