I agree - his mental toughness was the overriding factor. His handlers were concerned about his apparent lethargy while preparing for the ****ell fight. In the biography by Everett Skehan it notes that without his focus he would be vulnerable to a fighter such as Don ****ell.
Stamina, mental toughness, tolerance for pain, and power are long suits for Rocky. His hand speed wasn't bad either.
All the above and Luck. Came along at the right time, a weaker era with no direction after the drawn out demise of the division's greatest champ. Also, 10 years later I am not sure he would have been allowed to continue against Charles.
He has lots of things mentioned by previous posters in abundance but his most extreme facet I think was his conditioning ( I mean fitness, powers of revovery, stamina etc etc). Especially for a HW. Could have fought 25+ rounds I think at the pace he fought 15rds. Amazing.