Headlined by Bill Belichick at North Carolina, the ACC has three ex-NFL head coaches leading its football programs this fall.

Boston College coach Bill O'Brien spent time with UNC's Bill Belichick in the NFL. Here's what O'Brien said about Belichick's move to college.

But Belichick, widely regarded as one of the greatest—if not the greatest—NFL coaches of all time, is already noticing differences between players at the college and pro level. And it goes beyond the obvious age and skill differences that one would assume exist between the levels of football.