With the way that Chelsea won the European Club Competition, I think it has been easily demonstrated the importance of a solid back four and having great defenders who can stay compact in the defensive third. Granted, Chelsea played with more of a back 9 than anything to be honest, but Terry, Cahill, Ivanovich and Cole were absolutely vital to the European Champions' success.
Chelsea's success in the Champions League has led to a lot of people believing in England's chances at the European Championships in Poland and Ukraine. To be honest, England actually do have a top defensive unit to put out there, although they are weak in midfield and at center forward with Rooney out for the first couple of crucial matches.
The favorites for the tournament, Spain and Germany, both have questions in their back four. Per Mertesacker is not good enough to play at the top level in my opinion (not starting caliber at Arsenal, a poor defensive team) and he will be starting most likely for Germany. Spain are without Carlos Puyol and will have to rely on a recently-concussed Pique along with Sergio Ramos probably sliding in at right back, with Arbeloa and Alba as the full backs. One injury and they are in a precarious situation - if they are not already with a couple of second choice selections already starting for them.
The two strongest defensive units in the Euros are probably France and England. The FA missed out on Richards in my opinion, but will still be able to carry a squad of Terry, Cahill, Cole and the chance to choose from Phil Jones, Leighton Baines and Glen Johnson - having Lescott as a back-up center half isn't too bad either.
France have the luxury of a great fullback pairing - probably the best in the world - of Bacary Sagna and Patrice Evra (Gael Clichy), along with a half dozen center backs playing for top European sides. France, in my opinion are the most complete European side with no weaknesses at any area on the pitch.