I would say the world is too polarized and too peaceful to have too many heroes. Also, heroes are generally born after their heroic deeds have been done. Before 1991, not nearly as many people would have considered Nelson Mandela a hero. It was his work throughout the 90s that raised him to the status he has today.
In time, we might remember some people we would consider heroes - reluctant to name names, though I will throw Ai Wei Wei as an example. But progress up to this point has been the result of a group of people more than the actions of a singular individual.
EDIT - The # of people that might qualify to be heroes has been reduced as well. It's very rare for non-wealthy people to be remembered as heroes, and there are only a handful of such examples. I remember this one book from Rothkopf called Superclass, wherein he defines a member of a particularly select top-of-the-world social class (called a superclass) as a person who:
a. Is a multi-millionaire
b. Affects the lives of at least a million people outside of his/her nation every day
And, based on this criteria, there are around 6,000 members of this superclass in the world today. Well, less. Bin Laden died a while back.
Anyway, I just remembered it, and felt it was a good starting point in at least defining who may become a hero. These are, after all, the most powerful (not necessarily the wealthiest) people in the world.