What a wonderful question.
And what a bloody difficult question to answer, within the framework offered.
Photography is a mix of a number of diverse elements.
It requires a good physical knowledge of your equipment: all of it. How does it work; and how do the different elements of your kit interact with one another. Consider for instance issues such as what angles of view does a particular lens present to the focal plane, what range of apertures are available with a given lens have, how do those apertures affect your shutter speed selections ....
It requires a good understanding of light ... consider the quantity, quality, and colour of the light. From where is it falling? How and where does it create shadows? How will those shadows affect my image? What's the time of day? How harsh is the light, and how harsh are the shadows? How can I mitigate against any unpleasantness being caused by this light?
It requires a good understanding of your subjects: are they people? Perhaps flowers? Someone has mentioned motor vehicles; are they moving or static? what are their surroundings? within what environment will I be shooting them? What elements of that environment should be included, and how should that be presented. Getting back to the first point I've mentioned, how would a particular lens selection present this subject, and its environment, when compared with a different lens? And from what distance?
I'm only scratching the surface here, but with a good understanding of all of these points, and so many more, it comes down to how one, at the point of making an exposure, is able to take into consideration all of the appropriate elements and make them work, in a manner that enables them to produce an image that matches their vision of what the image should be like.
Simple.
