Which are in your opinion the factors that make a competitive NT?
Population of a country - There is more possibility to find talents. It is true for China, Russia, Brazil, USA, Japan, Germany, Turkey, Italy but not for India, Indonesia, Pakistan, Philippines, Vietnam, Mexico, Iran, France, United Kingdom. On the other hand countries with not so big population such as Cuba, Serbia, Netherlands, Azerbaijan, Croatia produce good players and have good NTs.
Income - Parents in rich countries can send more easily their sons to play sport. Strong clubs and championships are found in rich countries. That is true for Italy, USA, Turkey, Netherlands, Japan, Germany, but not for United Kingdom, Norway and other Scandinavian countries, France, Canada, Australia. On the contrary China, Cuba, Peru (of the 80s), Brazil (in the 80s and early 90s I don't think it was a rich country) which are not rich countries have and had succesfull teams.
Physical structure - Tall and strong players are important in this sport. Some countries have taller and other smaller average height population. That may be true for Russia, China (tall more than tall), United States, Cuba (strong more than tall), Serbia, Poland, Netherlands but not for Scandinavian countries or Jamaica (I think the girls are similiar physically to Cubans ). Countries with smaller average height are also succesfull such as Brazil, Italy, Turkey, Japan
Political structure - It is interesting that communist countries were succesfull in this sport. Maybe it has an influence in the method of training. URSS, Cuba, China, East Germany. And now post communist countries are succesfull too Russia, Poland, Serbia, but not Romania, Hungary, Czech Republic.
Popularity of the sport - Of course Brazil, Italy, Russia, China, Cuba, Serbia, Turkey, Japan are succesfull because the sport is popular among girls so they pick volleyball when they are little. But Nastja said that in Germany volley is not popular. I don't think it is popular in the USA either. But the teams are succesfull. In Peru and South Korea volley is popular but NTs struggle.