While Napoleon was most likely egotistical I think that a word to describe him would also be a visionary. Napoleon had a vision of what he thought the world should be and strived to teach that vision. One quote that is credited to him is "imagination governs the world".
In my opinion, what made Napoleon great is the same thing that makes most great people great -- ambition.
This is not an entirely positive characteristic, though. It was ambition that drove him to conquer other countries and to try to impose his own system on them.
Clearly, he had some sense of justice since he imposed a decent system on them (unlike, say, an ambitious man like Hitler). However, his main trait that made him great was his driving ambition.
Napolean was said to be arrogant and egotistical. His need for power drove him in many aspects of his life, and like many, he was only happy if he was victorious. There are some who say while he expressed these characteristics quite avidly, he was at the same time concientious of helping others. Like most people there was probably some good and some bad, and not everything history teaches is entirely accurate.