The element of the unknown. Bill Belichick has referenced it often ahead of his team's Week 1 matchup with the Dolphins. It's one of his standard talking points before the regular-season opener annually.
"It’s opening day, nobody’s really shown anything," Belichick said this week. "We’ll see what happens first game. That’ll declare a lot more going into the second game. That’ll declare a lot more going into the third game, and then there’ll be a point that we get a pretty good feel for what the other team’s going to do, and probably what [we're] going to do, too.