Developing the ability to reason: The ability to discern truth by an if/then process using only logic. The ability to reason is far and away more valuable than a traditional education alone. A formal education is wonderful to have, but is no guarantee that the student will learn to think and reason for himself. The world is full of educated fools.
Most people rarely develop the capacity to reason because of the human tendency to take the path of least resistance. Even with the educated, the intellect will almost always follow emotional bias rather than rational. People will choose what feels or seems best, rather than what makes the most intellectual sense.
It is just human nature to hear an opinion, agree with it, and count it as original thought. Someone will hear an argument, think it sounds reasonable, and accept it as his own, without ever questioning either the source or the logic. Much, much folly is perpetuated by this weakness. To become and remain successful, you will need to cultivate a mental posture of reason.