What is the single most differentiating factor that made the successful person very good at what they're doing? (quality, skills, habits, characteristic or personality)

In my opinion, I always practice IKIGAI (a reason for being), Ask yourself, "what you do and why you do". Set a goal and focus and zoom into the target. Working through the passion and identify your skills/talent, then move to the next step to add value and "GIFT" without harming others including the environment and humanity. Lastly, focus on the value and make sure is flowing with the integrity and allow everyone to claim the value and move up to the next level. Successful people know how to synchronise all of the above within the ecosystem.

Dedication - no one is master in the domain but continuous learning and improvement to reaching the mastery level. To be successful all you need to do is keep trying, failing, improve, and repeat.

Of course practicing good routines, becoming your habits, which shaping your characteristic and slowly evolve your personality could be a good quality change.