Galah, you're asking a bit of a trick question, the answer to it is 'Quality'.
Have you been reading Zen and the Art of Motorcycle Maintenance?
Things that people have listed so far include originality, technical proficiency, connection to subject matter, connection to the viewer. Reducing or defining Quality, in Pirsig's sense of the word, in a set of criterion like this is by nature incomplete and reductive, as these very values are derived
from Quality and not the other way around.
Show a gallery of images to a group of people, and there will be some common agreement which are great and which are not. There will also be others which some hate and others love. Each person will have their own reasons. You may recognise Quality when you see it, and you will definitely know when it is not there, but this does not make Quality definable in a strict intellectual sense.
Whatever it is, I know it when it's missing, by my frustration; remembrance of the sublime satisfaction of seeing it now and again keeps me burning film, against all odds.
How can this seemingly paradoxical concept of something undefinable actually help you when you are out taking photos?
Understand your tools (camera, film, darkroom) and your art. Use them until you know how they function and work so well until they become a part of you, an extension of your existence. This is what some call being 'in the moment'. There is a patience, care and attentiveness to what you are doing but it's more than that. It's a sense of inner peace where your tools and thoughts change together in a progression of smooth even changes until your mind is at rest at the exact instant the material is right.