I read today a post by Jeremy Dean that lists the factors that make a great leader.

A good leader has these four factors (Hogan, Kaiser, 2005):

1. Decisiveness: in case of uncertainity, they make decisions and take responsibility.
2. Competence: they are skilled and can create good teams.
3. Integrity: they are able to create deep trustworthy relationships.
4. Vision: they are able to see both short term and long term, seduce and inspire.

But to reach the next level and be a great leader, there are two more (Collins, 2001):

1. Modesty: it’s very interesting to see that modesty is a quality for great leaders, something that goes agains the common perception of being show-offs.
2. Persistency: they constantly push their goals, vision and push the team forward.

Jeremy wrote also another post, about the seven reasons why leaders fail. These are:

1. Strict hierarchies
2. Poor decision-making
3. Huge pay differentials
4. Impossible standards for leaders
5. Climb the greasy pole
6. Psychology of followership

These may all sound like pretty straightforward characteristics, but apparently few have what it takes. Many surveys have been carried out asking people what they think of their immediate bosses. On average these find that about half are seen as incompetent.
— Dean J. (2012) How to be a great leader

I did a quick summary here, but the two posts from Jeremy digs into much more detail: How to be a great leader and Seven reasons leaders fail.