A couple of things, according to Aristotle's Poetics:
"1. The tragic hero is a character of noble stature and has greatness. [...]The character must occupy a "high" status position but must ALSO embody nobility and virtue as part of his/her innate character.
2. Though the tragic hero is pre-eminently great, he/she is not perfect. Otherwise, the rest of us--mere mortals--would be unable to identify with the tragic hero. We should see in him or her someone who is essentially like us, although perhaps elevated to a higher position in society.
3. The hero's downfall, therefore, is partially her/his own fault, the result of free choice, not of accident or villainy or some overriding, malignant fate. In fact, the tragedy is usually triggered by some error of judgment or some character flaw that contributes to the hero's lack of perfection noted above. This error of judgment or character flaw is known as hamartia (αμαρτία) and is usually translated as "tragic flaw" (although some scholars argue that this is a mistranslation*). Often the character's hamartia involves hubris (which is defined as a sort of arrogant pride or over-confidence).
*ed.note: indeed, in ancient Greek it simply means "fault", not sin.
4. The hero's misfortunate is not wholly deserved. The punishment exceeds the crime.
5. The fall is not pure loss. There is some increase in awareness, some gain in self-knowledge, some discovery on the part of the tragic hero..."