I saw an interview this a.m. with Russell Wilson, and he was asked just what makes playing the Pats so hard. And he replied with what I've always said - it's the execution.
Bill is able to show (coach) players exactly where to be on the field, how to mesh with other players, how their role fits in the team, etc.
Just watch the Pats' plays, such as screen plays. The defense doesn't know it's coming, most times, and the receiver is always open. After the catch is made, the blockers are in perfect position, squared up on the defender or able to push them out of the path of the receiver, they engage just the guy they're supposed to block, pick up unblocked players, etc.
Teams all run the same basic plays, like sweeps, off tackle runs, screens, slants, post patterns, curl routes, draws and so forth. They do zone or man to man, or a combination of both on defense. Blitzes can be disguised, but only to a point. Everybody knew the mid-90's Cowboys were going to give teams a heavy dose of Emmitt and that Irvin was going to get a lot of throws, but they still won SBs because those plays were executed well. Most drives that offenses do don't fail because of great defensive plays, they fail because the throw is off, the receiver doesn't run the exact route, or drops the pass, or the quarterback misses the throw, and so on. If the defense fails, it's because somebody covered the wrong receiver or wasn't where the runner was going, or the offensive player can't handle the blocker. Sometimes it's sheer physical ability, but like on slant passes, they are near impossible to defend if the route and the throw is correct, because the receiver makes his cut and there's a half second or so before the defender's brain can see and react to the play, which allows the receiver to get a foot or two separation and catch the ball before the defender can get there.
Coaching matters, and the Cowboys are lacking in too many areas of that....