The key seems to have been Genghis Khan's unique value system:
"The greatest joy a man can know is to conquer his enemies and drive them before him. To ride their horses and take away their possessions. To see the faces of those who were dear to them bedewed with tears, and to clasp their wives and daughters in his arms"
Preferring rape and conquest to hunting and falconry, coupled with building an empire and "a social legacy that benefited his sons' sons unto the seventh generation and even beyond", meant that Genghis' progeny multiplied explosively, and his apparent Y-chromosome lineage today features prominently in the population genetics of Asia.
In "The Genetic Legacy of the Mongols" (Abstract|PDF), to be featured in the March 2003 issue of The American Journal of Human Genetics but which has already been published electronically, the authors report their discovery of the aforementioned Y-chromosome lineage, which due to it's age (~1,000 years), place of origin (Mongolia), and rapid spread, must in all probablity be associated with Genghis Khan or one of his immediate forebears.
Though absolute proof that the lineage in question is Genghis Khan's awaits the recovery of his remains and successful sequencing of his DNA, the only other possible explanation is that Genghis Khan did not spread his genes while some unknown man living in the same place and time did. This is unlikely, to say the least, since the enormous reproductive success of Genghis Khan's descendants is well attested in the historical record.
In fact, as we learn in Steve Sailer's UPI write-up of the study ("Genes of history's greatest lover found?"):
Incredibly, as late as the early 20th century, three-quarters of a millennium after Genghis Khan's birth, the aristocracy of Mongolia, which was 6 percent of the population, consisted of his patrilineal descendants.
Sailer does note that "population genetics is still a growing field", leaving open the possibility that a challenger will emerge to Genghis Khan's status as "the most successful patriarch of all time". Gregory M. Cochran, interviewed by Sailer, implies that Mohamed is among the very few historical figures who could potentially equal or exceed Genghis Khan in number of patrilineal descendants.
Another interesting aspect of the study is its apparent confirmation of the origin story of the Hazara, a tribe from Afghanistan believed to be descended from a Mongol army. While some have claimed there is insufficient evidence to support the Mongol origin of the Hazaras, the recent study all but proves the theory correct, with more than a quarter of Hazara males carrying the probable Genghis Khan Y-chromosome.