I rate yaya as being in the top three players at the club (possibly behind Kun and Dave) but there are more than a few moments where he frustrates me.
Take the first goal we conceded against Swansea as a case in point, and watch it back in slow motion if needed. They start to threaten with our back four fairly well organised and in a line around the 18 yard line (Swansea have the ball outside our area). Yaya and fernandinho are in front of our defence, presumably to provide protection, but Yaya applies no pressure to the man on the ball and allows him the time and space to think/cross/shoot/make an easy pass. So Nastasic then feels that he had to step out of that defensive line to close him down. By this stage however, he's had that extra second to decide what to do, we now have a defender out of position, Kompany (who looks torn between coming and going) has to come across to cover Nasty and Zaba has to do the same to cover Kompany. This split second moment of adjustment ends up with Kompany being in no mans land and missing the ball, Zaba making half the challenge that Kompany would have made on the goal scorer if in their regular positions and 'all' because Yaya didn't apply any pressure to an attacking midfielder, on the ball, about two yards away from him.
This really happened, maybe people won't see it the same or can explain further nuances of the situation to me but I'm not blindly criticising Yaya. He fails on his defensive duties (without the ball) on a number of occassions each game and without any change we will continue to concede goals that we dont need to.