With Malaysia being all about heat management, Williams have added new gills on each side of the cockpit to provide additional cooling for the car's internals. Even though this in itself is nothing new, it's interesting to see that Williams appear to have put in some effort in making the fins efficient, arranging them in two rows of variable sizes. It makes us suspect the team won't limit their use to a single Grand Prix.
Note that the area containing the gills is more or less the only possible location for teams to add apertures in the sidepods, apart of course from the rear end hot air exits and the 5cm closest to the car's floor. The latter area is usually not preferred as clean airflow in this area is often required for good efficiency of the diffuser.