Quality of light
Almost all photographers know for a fact that the essence and quality of light in a picture is one of the most important of its elements. Indeed, bad light could ruin a shot and good light will surely enhance it. The main difference in the quality of light is its definition of "diffused" or "directional". To me, useful light means diffused light. Be it a long exposure shot or a distorted one, I always prefer to shoot under cloudy/overcast conditions. Apart from the technical issues that are closely related to the type of light, to me, working with diffused light is very important when I setup my composition and framing.
It helps bring out an evenness and a sense of continuity in a photo. There are no harsh shadows that could hide/compromise important elements or work as distracting factors. You can see the different elements in a picture coming together evenly and undisturbed as the tonal range becomes more fluid. Also, diffused light helps to create a moody atmosphere in a photo, which is almost always my intention. So, light should be seen as one of the most important creative tools when setting up a photo. The very definition of photography is "writing light", so by being selective on the lighting conditions is what we ought to do when we decide to take a shot (or not). In black and white photography, diffused light becomes very useful when we need to alter the presence of the captured colors. I've faced many situations where directional light totally killed the quality of a painted element, both in terms of colors and "uninvited" lateral shadows. So, light needs to be studied, respected and cleverly used as no equipment, as expensive as it might be can replace the natural light in a frame.