Great Images vs. Mediocre Image

What differentiates an average, or even mediocre, photo from a great image? A great camera? A high megapixel sensor? The fame or notoriety of the photographer?
No. None of those things. The two most important aspects of a great image are a well-defined subject, and viewer engagement. That assumes the photographer has a decent grasp of the technical basics. A poorly focused image will lose its impact, regardless of the subject and engagement value.
An image creator can use several techniques to define the subject, such as depth of field, contrast, and, to a lesser extent, vignetting. Using a shallow depth of field is effective because an in-focus subject against an out-of-focus background will automatically stand out.
Contrast can mean either physical contrast, such as color contrast, such as a person in blue clothes against a red wall, or a brightly colored flower or plant growing out of a crack in gray concrete. Or it can mean metaphorical contrast, such as a lone woman standing amongst a crowd of men.
Vignetting can be effective, but it tends to be overused. Simply put, a vignette is a post-processing technique in which the outer edges of a photo are darkened, leaving the subject in a brighter oval in the middle of the photo. Without an image, it would look like this:

With an image, it looks like this:

The images that work best with vignetting are portraits, wedding photos, and photos simulating an old-time feel, such as those emulating photos of the Old West. Otherwise, if the subject is well-handled, vignetting comes across as a little heavy handed.
Viewer engagement can be a little trickier. The subject itself can be of more interest to some viewers than others. And, considering that the media inundates us with images, good and bad, all day, some viewers might not take the time to really look at a good photo. But, a good photographer can make any subject more interesting; an image of a common coin or a brick can be interesting if handled well.
The best starting point for engaging viewers is to pay close attention to the rules of composition, such as the rule of thirds, the golden mean, the rule of odds, etc. And, I use “rule” here loosely. I consider these rules as good starting points, almost as if they were “strong suggestions,” rather than “rules.”
Another aspect of engagement is angle of view. If the subject is an animal, or human, try to go for an eye-level shot, if possible. As viewers, we get drawn into the eyes of the creature, regardless of the species. So, if your subject is a child, take the shots from the ground, or a seated position – whatever it takes to get your camera at the level of the child’s eyes. If you’re taking pictures of ducks on a pond, get dirty, and get down on the ground to get to the duck’s level.
If the subject is small, such as flower, get close. Fill as much of the frame as possible with that flower. Leave no question in the mind of your viewer that the rose, or iris, or petunia is the subject of your image.
A well-defined subject and viewer engagement are interrelated concepts. You can’t have one without the other. And, when you have both, you have a good, or even great, image.