Not considering taste here? Oh... that is probably the only real answer...
Does it have a good colour or grayscale shades? Is it in focus, is it not blurry?
Do those technical details really matter?
I have seen out of focus photos where the main subject is so unique or dramatic that those details don't matter. This one comes to my mind:
In my opinion, as in all the arts, taste and opinion are the only thing that at the end matter. A photo of my baby is the best baby photo of all time!
Of course there will always be people that think that a really dumb or disgusting proportion is art.
I'm adapting a quote from Johann S. Bach:
“The aim and final end of all arts should be none other than refreshment of the soul.”
"A good photo should say something, a story, a feeling, a posture, a point of view, something new, something old. Perhaps there is a case that all that a good photo speaks is silence."