Joe Louis was a very hard puncher and fairly quick for a heavyweight, but often dropped his left and was a sucker for a good straight right - a la Max Schmelling. Marciano took punches well - seemed to shrug most of them off - and a very hard puncher, especially inside. Biggest factor is that he was relentless. In imagination, it would have been a good fight, but I think he would have taken Clay out during a 15 round fight.
