This is an eternal question and if the answer was simple we would all be doing it! Some people do have an eye for a good image and some simply dont - but what can you learn??
It is a question i get asked when im judging at clubs and you just know a great image when you see it and studies at MIT have shown that we make this decision in Milliseconds. So we decide intuitively but how doe we construct and capture that image?
Personally I feel images must have the following
Technical compitance
Balance
Depth
A purpose
Drama/statement/journey/story/illusion - as many of these as you like
It must enage the viewer and make thework in positive way
Lets take alook at what some other folk have to say in relation to What Makes a GOOD Photograph