Over-processing

Image post-processing is most effective if it is either so subtle that a viewer does not notice it or, on the contrary, so dominant that there can be no mistake that it has been done deliberately. 

Having post-processing as the main ingredient of the image is challenging because it can compete for the viewer’so attention with other elements (composition, subject, etc), so that the effect of each of them will be reduced. 

On the other hand, subtle processing does not necessarily mean a small amount of it. In fact, as the digital technology continues to develop, I believe that the overall workflow will become more back-loaded, where creative decisions about composition, lighting, and perhaps even focus, will be done at the post-processing stage.

Having said this, it has been widely speculated that still photography would soon be replaced by grabbing frames from high-resolution video, but this proposition over-simplifies the relation between stills and video. The latter is typically shot at relatively low shutter speed to produce blur in the individual frames, which makes the motion look natural.

In any case, even if a photo is taken with heavy post-processing in mind, it is important to be aware of the general desired effect and where it can be achieved more effectively and/or efficiently during shooting.