I thought about calling assertiveness the golden hammer in management, leadership and teamwork but that is pure exageration. If you do not like to work with people at all then forget it. The ability to be assertive should only be applied by people who strife in a team for a common goal.
Assertiveness is a social and communication skill that summarizes everything we want on us and others in our environment. Especially in IT projects. It is a term formed by psychology, communications and anthropology.
Nice article on being assertive: Skills you need
More articles: Assertiveness
After reading several books about agile Processes, Teamwork, Communication, Management and Work, I realized that assertiveness pretty summarizes all you need in one term. Start of Disclaimer - A master of assertiveness should be able to achieve any goal and assertive people are successfull people. - End of Disclaimer.
The problem is the high entry barrier in being assertive. It is comparable to pursue the way of buddha in enlightement. You can pursue it your whole life and still end up as a cow in the middle of nowhere in your next life.