Defending, more than any other position on the pitch I believe is a TEAM EFFORT.
Players have to be in synch with each other. The defensive line has to move as one. If one defender is not disciplined and moves or doesn't move with the line, it can result in plauying the attacker onside and GOAL! This is one glaring problem our backline has.
I was watching the Chelsea vs Arsenal match on the weekend and the match commentator was saying how prior to the match, the Chelsea backline was warming up together. Moving as a unit, doing the same exercises, stretches, positioning, in unision, talking to each other.
I think this, more than anything results in a good defensive performance by the TEAM.
You can have an individually good defender but if the rest of the line is not in synch, goals like rain will pass.
Chelsea's backline is not in and of itself the most gifted individually. Terry is exceptional, but Gallas, Ferreria, & Carvalho are ok players. It is how they operate as a TEAM, their ORGANIZATION, in addition to having a very good backstopper in Cech is what makes them so difficult to break down in my opinion.
The Italians have done it for decades. Others are now catching or even surpassed what they have done. Italy & Italian football has always been defence first....and devised a TEAM method to play in such a manner. It is no secret that most of the world's best defenders in the past 50 years have been Italian. To this day, the greatest defender I have seen is the former Juventus and Italy captain, Gaetano Scirea. Amazing player. But would he have been as great had he had a different backline? Next on my list would be Milan's Franco Baresi.
In summary....a great defender IN TODAY'S GAME has to have the basics covered. Able to control, pass & move into space to provide his team mate with an alternative. He is also one who can deny the opposition time and space, is one who plays within the team structure, has the discipline required to adhere to the overall ORGANIZATION of the unit and can retain his concentration for the duration of the time he is on the field. He s also one who possesses the intangible of HEART and DESIRE.
In contrast, a poor defender is one who does not adhere to the ORGANIZATION of the unit, who frequently loses concentration, who does NOT have the basics covered, and who is challenged in the intangibles.