Literally, he didn't do anything bad, and was great at many things. He was quick, hit hard, durable, technical, smart, etc....everything you could ask for.
There's a reason the great Archie Moore lost to him three times.
EDIT:
Muhammad Ali once admitted that Ezzard Charles was one of his influences as a boxer. Ali admired Charles slick but powerful style of boxing which allowed him to dominate the light heavyweights and became a force at heavyweights.
The Wikipedia has these to say about Charles:
n 1976, Cincinnati honored Charles by changing the name of Lincoln Park Drive to Ezzard Charles Drive. This was the street of his residence during the height of his career.[6]
He was elected to the International Boxing Hall of Fame in 1990.
In 2002, Charles was ranked #13 on The Ring magazine's list of the 80 Best Fighters of the Last 80 Years.
In 2006, Ezzard Charles was named the 11th greatest fighter of all time by the IBRO (International Boxing Research Organisation).[7]
The "Cincinnati Cobra" was a master boxer of extraordinary skill and ability. He had speed, agility, fast hands and excellent footwork. Charles possessed a masterful jab and was a superb combination puncher. He was at his peak as a light-heavyweight. His record is quite impressive. Against top rate opposition like Archie Moore, Charley Burley, Lloyd Marshall, Jimmy Bivins, and Joey Maxim he was an impressive 16-2 combined. Despite being a natural light-heavy he won the heavyweight title and made 9 successful title defenses. Nearly 25% of voters had Charles in the top 10. Half of the voters had him in the top 15. Two thirds of voters had him inside the top 20.
In 2007, ESPN online ranks Ezzard Charles as the 27th greatest boxer of all time, ahead of such notable fighters as Mike Tyson, Larry Holmes and Jake LaMotta.[8]
In 2009, Boxing magazine listed Ezzard Charles as the greatest Light Heavyweight fighter ever, beating the likes of Archie Moore, Bob Foster and Gene Tunney.[9]
Prominent boxing historian Bert Sugar listed Charles as the 7th greatest Heavyweight of all time.
Thumbs up to Teodor, I cannot improve on his answer. Well done.
He could box and he could punch. In short, he's both skilled and powerful.
Was it his toughness and endurance? Was it his footwork and agility? Did he throw powerful jabs or hooks?
I'm just curious because there's not much information around about Charles.
