(Original post by Sir Fox)
He has done it to an extent not known before. Most conquerors in world's history had the intention of militarily conquering an area in order to eventually benefit from it, economically, strategically etc. (let's leave the crusades aside). They used violence but it was limited. After having conquered a region the Romans established their highly advanced culture, legal system, architecture etc. which eventually proved to be a benefit for all those who lived under their system.
The mongols conquered without establishing something new, the local population didn't benefit in any way.
By the way, his first quote
Genghis would ride up to a city, say "Surrender or you'll all die", and then, when they refused, keep his promise and kill them all. During his initial conquest of a region this would actually work quite well, because when news got out all the other cities would surrender without a fight and be spared (apart from the mandatory looting). In general he preferred to leave infrastructure intact to make the region easier to control, but he'd regularly use mass destruction as a fear tactic or to get revenge on someone who'd done him wrong.