D> For example, we have at least a basic article on every single town in D> the U.S. (thanks to the census database) and almost all in the U.K., but D> don't even have articles for every state or province of some countries, D> let alone all the cities and towns.
There are a lot of cities with more than 100,000 inhabitants for which we have no article...
Fun fact: there is a European city with 160,000 inh. having no article!
http://es.wikipedia.org/wiki/Alcorc%C3%B3n
http://en.wikipedia.org/wiki/Alcorc%C3%B3n
D> There are people who have been D> president or prime minister of a sovereign country for whom we don't yet D> have a biography.
Of course there are loads of them. Not only PM/Prez, but also a lot of kings and other rulers.
I can guess that the number of missing incumbents is around a couple of thousands. Many of them ruled countries of which the average westerner never heard. :-)