Well, I've determined what it isn't.
Allies of WWI: Japan, Italy aren't included.
Allies of WWII: Brazil isn't included, Spain is.
Most spoken languages: Portuguese should have more speakers than Tagalog, unless the Philippines is being counted as Spanish-speaking territory.
Extent of empires: Switzerland had no empire.
Countries that have fought a war since 1945: missing Korea, Vietnam, Angola and Mozambique, among others.
It can't be anything economic because tiny Malawi is in, but Germany and Japan are not. Same for anything related to social welfare.
It can't be flag colors because Iran would mean red, green, and white are out of the mix and those appear on others. Brazil with blue and yellow, Korea with black... leaving very few colors left behind. Nor is it their shapes.
... so, as the tree said to the woodsman, "I'm stumped."