From what I can work out, early response, clear messaging from leadership, early social distancing measures (early lockdown), and responsible citizens are the key factors.
I know Joe's other "theory" is population density - this does not seem to be as important as the above-mentioned factors however. The graph shows the countries with populations over 5,000,000 and I chose them because a few of them were mentioned and a few others I thought were interesting. From left - Taiwan has the highest density of people per square kilometre and Australia - on the right with the least density.(The blue line). The green bars are deaths per million.
Denmark, Taiwan, Germany, Greece, South Africa, Israel, Australia, and South Korea all appear to have been quick off the mark - the rest speak for themselves, I think.
The island nations, it could be argued, might be easier to isolate like OZ and New Zealand who have done well.... Taiwan seems to have been similar to South Korea in that their close proximity to China made them far more aware of viruses and thus far better prepared.
South Africa, as I have said before acted very quickly and pretty strictly too with the lockdown (including the prohibition of liquor sales!!).
Once again, I am open to criticism regarding my methods.