Practice accounts for around half of the difference between individuals.
Working memory capacity accounts for a much smaller, but still significant, amount of the difference.
It makes sense that WMC would be more important in the early stages of acquiring expertise, but research has found that WMC does not, in general, appear to interact with practice or amount of knowledge. However, there may be some tasks for which WMC becomes less important as expertise increases.
Expertise should also allow you to functionally increase your WMC through the use of long-term working memory.
Personal attributes such as self-discipline and motivation are likely to affect number of hours of practice.
Individuals vary considerably in how many hours it takes them to achieve the same level of expertise.
One important factor behind this may be starting age — if you start to develop expertise in a domain before adolescence, when your brain is still structuring itself, you probably have a big advantage (although that may be more true of some domains than others).
It does not appear to be true that ‘extra’ IQ points over 120 make no real difference to personal achievement.
Attributing some differences to ‘genes’ is a misnomer. Environment interacts with genes, and what’s important is what is expressed. IQ is certainly affected by early environment, and perhaps WMC is too.