I would concur with S4 - looks like a fairly aggressive attack has been made on the tone curve, resulting in "gaps" in tonal range. Also, 5 minutes removing some dirt specs from the sky with a clone tool wouldn't go amiss.
All that aside, I am impressed by the quality of what is, effectively a flatbed scan of a slide - given that the optical resolution is significantly lower than dedicated film scanners and, I assume, the resolution acheived in this image is through scanner interpolation - the result is remarkably good. But perhaps it is the interpolation process which is causing the "grainy" result?
Colin K. Work, Pixstel