I've come to believe that most people can be classified into 2 categories based on their concentration span. I assume that everyone in the Universal Set of people has something to offer to everyone around them, no matter how little it may be. So, the 2 broad categories based on concentration span would be:
1. Those who can concentrate on something for long periods of time.
2. Those who can concentrate on something only for a short period of time.
I have further noticed that generally, the product concentration(C) X concentration span(CS) is constant over a large set of people.
So, if C X CS is constant(K), then, it would imply that people falling into category  don't concentrate very hard on what they are doing, but do it for longer(marathon athletes), whereas those falling into category  concentrate really hard on short bursts(short distance sprinters).
I think these basic differences dictate as a person what job you are most suited for. For example, managers and overseers of tasks could fall into category  whereas inventors into category . Say you are working on building a large system. You would need someone from  to look after the general progress and well-being of the system, whereas you would also need someone from  to provide the much needed pushes at vital stages of the project when it is stuck or in need of an idea.
I think that if people try to cross these barriers and try to choose careers that violate this basic underlying trait of theirs, then they would find it very difficult to adapt to their job, and would feel under-qualified as compared to their peers who are from the suitable category of candidates.