Running backs have an instinctive skill that you cannot coach. A good running back can make the first guy miss. A great back like Eddie Lee could make 2 or 3 or 4 guys miss. Defense is the same. It requires instinctive skills that cannot be coached. A good defender 1. must anticipate the play before and as it develops, find the ball, and pursue the play; 2. must not get blocked easily and must not get focused on the blocker instead of the ball carrier (cannot play patty cake with the blocker); 3. must get to the ball carrier in a tackling position and deliver a bone rattling lick with wrapup (this has to be something the defender really enjoys...rattling bones). Tackling technique and mobility and strength can be improved with coaching but the basic instincts and especially the enjoyment in being aggressive must be present in the defender. (watch Paul Davis.) The coaches have to find those instinctive players. Yes, some offensive positions are more complicated and require more self discipline, but players can be coached to play the blocking offensive positions IMO.