Schemes, talent, "young talent", experience. I don't know enough to sort it all out. Even despite the injuries and defections, Rodriguez has three classes of recruits out there and they seem to be totally clueless at times about where they are supposed to be. Fundamentals like tackling, etc have been discussed ad nauseum.
As to how much GERG is responsible, I had the thought today that perhaps GERG is not a good "college" coach, despite his success in the NFL. The difference is that an NFL DC has nothing but extremely disciplined and accomplished professionals at his disposal. He does not have to coach fundamentals. If they don't have that already, they have no shot at the League. Scheme on the other hand is where NFL DC's separate themselves. Maybe GERG is good at scheme, but not able to communicate and teach the basic fundamentals of sound football to young players.