Most pundits who attempt this question focus on the personal characteristics of the developer as an individual. Accordingly, some say it’s about technical skills, some about abstract reasoning, some about the ability to work in a team, some about an improvement attitude, some about hard-working dedication, some about systems thinking… yadda yadda.
I have a different take.
Dr. Deming has taught us that 95% of the performance of an organization is attributable to the system (processes, technology, work design, regulations, etc.) and just 5% are attributable to the individual. If we accept this (and I certainly do), then all the discussion about attributes of the individual becomes essentially moot.
Take an outstanding developer from one system (e.g. team, business) and place them into another system, and their performance will depend almost entirely on the system they’re now working within. And NOT on their own personal attributes, skills, talent, or whatever.