McLaren has introduced a modified sidepod bodywork design that features a bump on top of the sidepod. In fact it looks like the fat lady sat atop of the sidepod, creating this new version. The idea is to draw a little bit more air down onto the exhaust more rapidly, helping the curving down of the exhaust gases aft of the exhaust channel. The difference of course is clear because of McLaren opting to paint less of the car's bodywork in silver.
The design is not new at all and follows a trend set by Mercedes as their F1W04 features a similar dent in the middle of the sidepod, creating a mild U-shape in the upper surface. Lotus have recently done a similar design by applying it on their ramp style sidepods.
As an aside, also note that McLaren ran without gills underneath the upper frontal wishbone, hinting at either modified internal cooling channels or simply because the Chinese GP weather posed less of a challenge than the burning heat of Malaysia.