Leadership is an important element for successful project managers.
A very useful study published in 1998 about program management  relied on a survey of top senior project managers. Though the study is a bit dated, the resulting list of 9 characteristics of an effective project manager is still useful. They are:
1) Leadership by example
3) Technically competent
5) A good communicator
6) A good motivator
7) Stands up to upper management when necessary
8) Supportive of team members
9) Encourages new ideas
The authors of the study say that project managers must “combine their technical competency with the application of proven project management tools that support project planning and control, and the need to practice leadership skills that are compatible with the internal motivation of the project team and externally compatible with client focus strategies.” The study certainly got this part correct.
I think the associated insights presented by the study authors are important since project managers are very often technical individuals (with engineering or other science degree) who sometimes struggle with interpersonal skills.
For these reasons, it seems to me that a good program manager must combine technical expertise with great personal skills in the organization and with the customer.
 T.W. Zimmerer, M.M. Yasin, “A leadership profile of American project managers,” Project Management Journal, Vol. 29, No. 1, 1998, pp. 31-38.