So, we all know the Mongol Empire. However, that empire was built over a long period of time (from Genghis Khan down to Kublai Khan, or about 1206-1279). So the empire is not Genghis Khan's personal achievement. In fact, I do think that Genghis Khan empire simply built on-top of the nomadic model of various other empires (says, Khitan to the East, Goth and such on the west).

That said, Genghis Khan was a magnificent person. In my opinion, his true greatness lies in:

1. Genghis Khan replaced cronyism with meritocracy. The steppe was ruled for generations with a rigid social structure where birthplace decided a person place in the society. Genghis Khan would have none of that. He promoted and rewarded for merit, strength, bravery, and loyalty. Moreover, he actively sought out for new talents. For example, he dug up Yelü Chucai from Jin, wrote letters to learned sages of Northern China, and sponsored new scripts (see below).
2. Genghis Khan (if Genghis Khan and the Making of the Modern World by Jack Weatherford to be believed) also christened his nation: Mongolia was first called as such by his Kulruitan.
3. Genghis Khan imposed a much more efficient social and military structure. Previously, as said, people were grouped by birth: in clans and tribes. Genghis Khan grouped 10 men into a group (squad?), 10 squads into a company, 10 companies into a regiment, etc. This, like the Chinese historians described, allows him to command his regiments like his arms, while regiments command companies like elbows, and companies command squads like fingers.
4. Genghis Khan also ordered and sponsored the creation of Mongol script. I frankly find this to be his most outstanding achievement: a nomad who understood the value of writing and record.
5. Genghis Khan unified Mongolia and the steppe. This unification is the foundation upon which the Mongol Empire would build. BTW, unification of Mongolia (and Northeastern China) was more complicated than first blush. Various empires arose from this area (by then, the most recent ones are Khitan and Jurchen), but none of them truly united Mongolia like Genghis Khan did.
6. Genghis Khan established religious freedom within his domain. This policy would continue for much of the Mongol Empire.
7. Genghis Khan actually cared about and rescued his wife. If my understanding is correct, the life for a minor man on the steppe was so harsh that he would abandon his women in case they were captured by larger enemies. Genghis Khan did not. Instead, he abandoned his then lifestyle and embraced world-conquest just for the sake of his love. That is just plain romantic.
8. Genghis Khan accepted Jochi (his wife's first born) as his own son in the face of massive doubt (lingering till today). This makes Trump and the charade of "manliness" within modern America look like overcompensating men. To give context, Jochi's inception happened roughly when his mother (Genghis Khan's wife) was first kidnapped. Obviously, as a young woman, she was raped. So, the question arose: was Jochi truly son of Genghis Khan? The great Khan refused to even entertain any other possibility, and included Jochi within his inheritance. Such is true manliness.
9. Genghis Khan genes are within about 50% of East Asians alive. This sounds freaking impressive.
10. More than a historical person (kinda like, says, George Washington or Alexander the Great), Genghis Khan has also become a myth, an inspiration, and story, and the father of his country.