There are so many things that go into making a great image: composition, subject matter, aperture, and much more. Yet I have discovered time and time again the main difference between a good image and a great image is one thing... LIGHT.
Lets say that you have your camera on a tripod and are photographing from the top of a mountain. First you shoot mid-day, then one toward the evening, then one at sunset. If you have your camera on a tripod, all images have the same composition, the same (or similar) aperture, and are fundamentally the same image. Yet it is likely that you have one terrible image, one acceptable image, and one outstanding image. Why? Because light makes all the difference. This is why Canon Explorers of Light have that name... that's what they do... they find the light, create the light, or wait for the light to create stunning images.
This is true in all types of photography. The light is fundamental in nature photography, photojournalism, fashion photography, portraiture. I've seen very strong images taken with average light-- there was something in the image that was redeeming like stunning subject matter. Yet I have seen few if any amazing images with average light.
If I could give any beginner photographer a piece of advice to improve their photography, I would tell them this: "Learn to see the light. Go to the Light." It sound like I'm preparing someone for death, but really its fundamental for a person's success in photography. If you learn to see the way light shapes a model's face, your portrait and fashion photography will improve drastically. If you learn to anticipate and wait for the right lighting in landscape photography, your images will improve drastically.
Here is an image I took on top of Cadillac Mountain at Acadia National Park in Maine. The image was taken in June 2005 with my Canon 10D and a Sigma 17-35mm wide angle lens. Why is this image not just a good image? It is a great image because of the light. I woke up at 3:30am to drive up to the top of the mountain with my mother. We set up our cameras, our tripods, and waited for sunrise. It was so windy I could barely stand, it was freezing cold, and I remember tears freezing to my cheeks when the wind blew them down my face. Yet I can say without a doubt, it was worth it.