In your opinion, what is the MOST important thing that people who want to develop their career in project management field really NEED to know?

1- People management.  One of the very important aspects of project management is how to communicate and coordinate with his team members.  Also communication with top management and stake holders and managing their expectations.

and

2- Time management to get the most in the least possible amount of time and to be able to priotrize.

I totally agree with Doha in her good answer.

People management and time management are the most important two.

80 % of Project Manager time is spent comunicating and influencing others.

So if he know how to communicat to team , staff, stakeholder and his organization he will success in project management field. 

This takes me to my specialized service in this specific area " Project ManagER Competency Assessment" (PMCA) service which aims at assessing and improving the competency of the project manager in three different areas ;  Project Management discipline, Project Manager character/personal merits, dealing effectively with project management work environment