There are several elements which create true heroism (from the ancient Greek perspective) in the Iliad.
The first quality a hero possess in Homeric epic is noble birth, and usually a lineage that may be traced back to a god. Although in the twenty-first century most people believe that anyone, no matter how poor or obscure their parents, can act in a heroic fashion, Homer tended to connect noble birth to having a noble nature.
Next, the hero was physically strong, and usually attractive as well, with physical prowess being an outward sign of good character.
The hero was trained to be a skilled "speaker of words and doer of deeds." Someone who was inarticulate would not make a good leader and thus would not be heroic in stature. The heroic figures in the Iliad are eloquent speakers and demonstrate great skill in combat.
Finally, the hero was pious, following the dictates of the gods. The true hero obeyed the laws of hospitality and acted with honor in combat.