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.
Best Answer
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:
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.