Good leaders are usually defined by their interaction with their subordinates, their reaction to harsh or extreme situations, and their ability for both compassionate and pragmatic decisions. In all good leaders, there must be trust between the leader and the workers; without that trust, someone will always try to usurp the leadership role. If a leader can win trust, the workers will want to perform at their best both for the leader's benefit and for their own. Another important quality is confidence; without the ability to make hard decisions and stand by them, trust will quickly evaporate.
One very important quality is integrity; the phrase "say what you mean, and mean what you say" sums up the need for a leader to be both honest and forthright, not lying for personal gain. Good leaders have personal opinions like everyone else, and know when it is appropriate to speak and when it is not. A leader who flips his opinions based on what he believes will boost his position will almost always be removed from his leadership position.
A good leader should be able to elicit the best possible work from his subordinates without driving them to exhaustion. The work of leading is itself stressful, but a good leader knows that he is only the figurehead standing on the hard work of his subordinates. Without the ability to inspire good work, the leader will simply be demanding and inspire resentment.
Finally, a good leader should know when to accept blame. Too many leaders work from a top-down perspective, where they take all the credit for work and place all blame on their subordinates. A good leader says "If we fail it is my fault, and if we succeed it is because of my workers." This is vital, and if the positions are reversed, people will leave because their work is not appreciated.
Two leaders that fit the above definitions are: Abraham Lincoln, who worked to unite a country at war with itself and had no expectation of personal glory or gain, and Winston Churchill, who was able to inspire greatness in others by the virtue of his words and actions.