I thought about some of the same things when I glanced over this thread before. I think a lot of success of the fighters that were known for the toughness is partially due to their ability to avoid getting hit cleanly, and simply being more aware. Even though a lot of guys probably don't consider Jack Dempsey or even Rocky Marciano defensive fighters, one thing that should be noted they were hard to hit cleanly. This was said by accomplished opponents of both of these men. Gene Tunney worked on developing his right hand after he saw Dempsey rocked by a hard right from Carpentier, yet Tunney couldn't land it cleanly on Dempsey, and Joe Louis remarked that Rocky was young, strong, and hard to hit. This should cause one to wonder what made them hard to to hit with a good punch. Rocky apart from his conditioning wasn't blessed with a superior "chin", Archie Moore, and Jersey Joe Walcott proved that. I believe that his posture, his semi-crouch, along with all the help that Charlie Goldman brought is what made a difference.

On another note look at what happened to Tyson as time went on. He had the toughness, he had a neck and legs that were as thick as tree trunks, but as time went on his posture got worst, his feet got wider, his game plan became dependant on just going in and landing a bomb, and consequently he became easier to hit.