I am uncomfortable with describing folks as "great," or "mediocre." I feel that simplifies people too much. We're all ongoing epic sagas; even the most banal people in the world.

One example is incompetent people with extreme willpower. They can Make Shit Happen; even if it is not good stuff. I think we've all had bosses like that. Some folks will just jam beans into their noses[0], and, for whatever reason, they are in a position to impose this on others. Sometimes, the bad stuff is actually a trigger for growth. I doubt we'd have much of what we have now, if not for a couple of genocidal wars, last century.

Others can be insanely competent, but are never in a position to apply that competence to cause any meaningful change.

Also, never underestimate the power of personal insecurity. This can be a huge driver, and substitute for willpower. People spend millions, trying to salve personal insecurity, and, likely, many tyrants (and great leaders) have been driven by personal insecurity.

[0] https://archive.uie.com/brainsparks/2011/07/08/beans-and-nos...