I read an interesting discussion about what makes a Project Manager outstanding? After reading this, I still hold my position that I am holding for many years now : a Project Manager should make every stakeholder happy. Including the PM self. The better a person can reach this goal, the better Project Manager he is. Being able to achieve this, a person needs very good "soft skills", not only pure communication, but also the ability to get into other peoples "worlds", understanding why they operate the way they do. And although this is a controversial topic, for software projects, the PM should have a good knowledge about the underlying technology. Otherwise effective communication becomes more difficult; it is an additional handicap when a different (in this case technical) language is used.

Well, that is just my summary. Take a cup of coffee and read the entire discussion.