There are many skills, attitudes and behaviours which make up a project manager. The APM Competence Framework is a great place to start.
In our experience these attributes make the difference between a PM and a great PM:
the ability to quickly move between the detail and the big picture,
awesome communication,
a sense of urgency,
great organisation and structural understanding,
emotional sensitivity, and
being an inspiring leader, able to infect the team with passion and enthusiasm.