Greatness begets fame. Case in point is the example I used here. Marcus Maecullus was a Roman General who achieved a great deal. He fought 39 battles across the Mediterranean, winning vast lands, and later went on to have a great political career. His greatest claim to fame is as the person behind the murder of Aristotle. In his day, he was feared and hated by enemies of the Roman Empire just as much as Ozai was hated and feared by enemies of the Fire Nation.
So what is the difference? Why is Caesar still famous today, and Maecullus forgotten?
The answer is that when all is said and done, Maecullus simply did not embody "greatness". He was an average tactician and general - all he really knew how to do was crush his opponents with sheer numbers, preferably while he was sitting a long way back from the front line, just giving the orders.
Then you have Caesar, a warrior at heart who loved nothing better than charging in at the head of his men to fight on the front line. He was a brilliant tactician who couldn't care less about having an advantage in numbers, sometimes heedlessly rushing in to attack enemy armies that outnumbered him 10 - 1 (needless to say, he still always won). His political career was far more dynamic than Maecullus's, obviously.
See the difference? It's all about skill and style, as well as what the accomplishments lead to.