I stole the wording of this question from /u/theamazingmrmaybe who put it much better than I did in a previous post where I asked the question but broke a subreddit rule (sorry!). He explains that if going by the "great man" theory of history that he is possibly "the greatest figure in all history" and goes on to describe him as a genius in the same strain as a Mozart as a musical genius, but for Caesar his genius was for "what he wanted to do" which was to rule Rome. So, to put it succinctly, what made him so great?
His cult of personality for one. Caesar was a remarkably charismatic man and his soldiers and the people loved him as dearly as any leader was ever beloved.
His brilliance on the field of battle is another thing. He blended a great deal of qualities together. He was an excellent tactician with a supreme eye for ground. His larger picture vision and ability to execute it ranks him amongst many a great strategist. He was down to earth and often suffered through many of the same discomforts as his men and rarely required of them things he would not do himself.
He was a fairly decent politician, negotiating himself through a myriad of odd alliances and friendships such as the First Triumvirate. His eye towards the common people and various reforms often offended the upper classes, but it endeared him to the people and his soldiers whom he strove to take care of.
All in all Caesar ranks as a remarkable blend of charismatic icon, general, and statesmen. What makes him different and among the greatest men of all time is his ability to excel and thrive in each of those roles. He is not unique, but few ever accomplished as much as he did or left as significant a legacy as he did upon his death.