The sharpness of the lens, high resolvability of the sensor and smart noise and JPEG processing all contribute to the X100S' excellent JPEG quality as high as ISO 1600 in good light. The extra-thin optical low-pass filter on the sensor does result in some rendering oddities, though; notice how the lack of antialiasing distorts the text.
While the lens generally has extremely good center sharpness and reasonably good edge sharpness, f2 is close to unusable. You can see the huge difference simply between f2 and f2.8 in the center; the outer edges become decent at f4.
The X100S' handling of bright, saturated reds and pinks is better than the X100's, but there are still some issues with hue shifts in the Standard default color setting (Provia film simulation). Switching to anothing setting prevents the orange areas you see here, but there are still some areas which render clipped. Additionally, these aren't quite the right color in any of the JPEG settings.