The million dollar question: what makes a great photograph great?
If the answer was obvious, there would be many more great photographs out there. Most photographers create lots and lots of good photographs, and they create a handful great photographs.
Great photographs are elusive.
What makes a photograph great? You'll get different answers if you ask photographers, art critics, and art collectors. Not only that, but different photographers will have different opinions, different art critics will have different opinions, and different art collectors have different opinions.
There is no one answer and no easy answer to the question.
There are, however, some common threads between most great photographs. There will always be exceptions, but by-and-large these things are found in great images:
Technical Excellence. Meaning, the shutter speed, aperture, ISO, depth-of-field, sharpness, etc., were all exactly as the artist wanted them to be and exactly where they needed to be. Not all technically excellent photographs are great, but all great photographs are technically excellent.
Strong Communication. Photography is a type of non-verbal communication and all great photographs clearly convey a thought or emotion. If a photograph says nothing, it will never be great.
Drama. A photograph can speak a thousand words, but if there is not some tension or dialogue, then those words are wasted. Use light, contrast, interaction, etc., to your best advantage.
Innovation. There are many so-called "rules" in photography, which, if followed, ensure consistently good images. Great images almost always break these rules and push the envelope of what is acceptable. Think outside the box and color outside the lines. If you never break the "rules"of photography, you will never have a truly great photograph.
Simplicity. The best photographs are often those that are seemingly simple. Crowded images tend to send mixed messages and cause confusion. Reduce the scene to its most basic elements, then open the shutter.
Time. Great photographs don't come often, and they usually require much effort and many attempts. If a great photograph doesn't happen--don't give up! Put in more time, effort and practice and it's bound to come.