He has poor positioning and is error-prone. Hopefully he is grateful to have another error-prone defender alongside him to take most of the heat. A good example was when either Luiz or Ivanovic (don't remember which) were shown red against United last season after RVP beat Cahill with ease. He also had a terrible play for England when they played Brazil recently that ended in a goal.
I read an article on Chelsea's centre-backs a few months ago. If I recall correctly, the side gave up more goals when Cahill started than when any other combination of Terry/Luiz/Ivanovic started.
This post was edited on 7/11 at 8:09 pm