Mathematics
In 1007, Europe is still using Roman numerals. A handful of intellectuals understand Arabic numerals — they were introduced to Christendom by Pope Sylvester II just a few years ago, but were not embraced. Even in Moslem lands, only scholars use Arabic numerals (though even ordinary Moslems, who use Babylonian and Hebrew numerals, have a great advantage at math when compared to Europeans)!
Roman numerals make mathematics difficult. Simple addition and subtraction can be done on one's fingers, but any calculation more complicated than that requires an abacus. With an abacus, complex arithmetic can be done — even square roots. Still, a lot of the simple math we take for granted today —the simplest of algebra, for example— remains impossible.