For anyone smart enough to not click this, tl;dr: US passports are 98.4% as good under their metric, and 6 other European countries are 99+% as good.
(Germans get additional visa-free access to 1-3 additional countries)
Is there any data available on the countries each passport offers visa-free access to? I'd really be interested what the few countries are that the first few pasport ranks differ between.
Seems like a much better metric would be total population that can be accessed without a visa or tourism volume access without a visa rather than just a raw count of countries.
I would guess this is mostly a function of the wealth of the country, and the map seems to support this.
States are more willing to let (likely-to-be-) richer people enter than poorer people.
Also contentiousness and aid-giving of countries. Germany doesn't declare war, and only does any kind of war through NATO. It also gives a lot. Both of these apply to all the high scoring European countries. For USA it's probably the aid and quid pro quo or giving visa-less or passport access to the country that the Americans can now get into.
The firm behind the study made a pretty interesting infographic: http://visaindex.com/