I've been thinking about what one skill can really set a project manager apart from the others. If we use the axiom that perception is reality (and in fact, perception is probably more important than reality), there is nothing more important than a project manager being "perceived" as competent.
And the best way to be perceived as competent is to be able to present well, and to address tough questions with poise. That sounds a lot like a Public Relations expert, doesn't it?
I'd argue that if a project manager isn't comfortable speaking to a large group and addressing questions in a polished manner, he or she is operating with a severe handicap. After all, this is needed to conduct project kickoffs, facilitate WBS development sessions, make presentations to stakeholders, communicate bad news, defend mistakes, and a host of other areas. And during any of these, a project manager needs to be able to address the skeptics with ease and polish.
No doubt, the best thing a project manager can to appear competent is to build presentation skills and learn fundamental PR.
To get started, here's a great article on how the PR experts handle tough questions...
Handling Tough Questions: Agility Versus Force