With the advances made on the X's and O's side of things, I believe that the biggest difference between today's athlete and athletes of the past is football IQ. It seems that a lot of todays players have a deeper understanding of the new offenses and defenses. And, that comes from coaching. Teams' attacks are much more intricate than they used to be and that, in my opinion, is the biggest difference.
Conceeding the fact that players are bigger, faster and stronger, I believe that players are just smarter...in terms of football IQ.