They understand their Customers better than anyone else
The top product managers truly understand their customers. Often way better than even their customers understand their own situation. They are not only able to understand requests & requirements but are also able to innovate for their customers and market their creation.
See: https://www.blossom.io/blog/2011/12/23/empathy-focus-impute.html
They are able to communicate what they know to their Product Team
They are efficient communicators. They understand that product teams crave purpose & context to make great trade-offs. They ask "Why, Who cares & So what?" and help everyone in their team to understand.
See: http://37signals.com/svn/posts/3392-humans-need-to-know-why#all_comments
They are cross-functional
They are unicorns. They are holistic. They can zoom out & drill down unlike others. They have a strong background in many fields. Design, Marketing, Engineering, Statistics, Sociology, Psychology, History & many others. Always curious and eager to understand & learn more.