From Ross’s great analysis of Ohio State’s offense (and the lack thereof) at Along the Olentangy:
Every primary backfield action needs to threaten all 11 defenders. What a primary play needs is good counter plays. Every defender needs to be worried about the ball coming to his area – on a throwback screen, a reverse, a play-action pass, or whatever – as a play begins.
What makes a defender good is something to read. If he can say to himself something like, “As soon as that quarterback makes that half-assed fake, I’m going to find the tightend coming across and try to get an interception,” if he can read initially and react accurately, he can play over his head. Counters, not mirrored primary plays, keep defenders from reading and jumping on plays.