To paraphrase Neitzsche, "Leadership is dead and we killed it."  Having spent a great deal of my adult life reading, studying, pondering, and testing leadership theories, I can honestly say we should stop thinking/talking and start doing.
One of the questions I ask in all of the MBA classes is the following, "Of all the great leaders in the history of the world, how many were graduates of a leadership development course/class?"  I think we all know the answer...ZERO.
There is only one way to learn to lead and that is doing it without a net.  You have to be willing to put yourself out there and risk it all in order to lead.  As the great John Wooden said, "It is what you learn after you know it all that counts."
I think the challenge is that great leadership can only be evaluated in retrospect.  We almost never identify it while it is happening and no one really makes the conscious decision, "I think I will perform leadership now."
As Supreme Court Justince Potter Stewart said in Jacobellis v. Ohio (1964), "...hard-core pornography is hard to define but I know it when I see it."  So it is with leadership as well.