What are the most important skills to have using Photoshop / GIMP / Darktable?

Restraint. Yes, you can do some amazing things with Photoshop, but the basic rules of light, color, point of view, and composition will always apply. Mind bending images may get noticed but the eye quickly tires of them if they are not artistic. My friend who created my website/weblog image is a true artist with Photoshop. He composed this image by piecing together disparate images and then giving it a patina that looks more like a painting than a photo. Other professionals have been begging him for a description of how he accomplished it.

Learning the software, what you can and can't do with it. It all starts with a quality image, if you don't have a good image to start with, nothing you do will fix it. Used to have to drill that into my new troops, it all starts when you hit the shutter release.

A strong image to start with. Old saying goes, garbage in, garbage out