On a philosophical level, I agree that "good" is so subjective as to be undefinable. And I know from experience at my monthly critique group meetings, that (as DrTang mentioned earlier) for almost any objective critique given, another person in the group will disagree, and say that the feature of the image which one person objected to is exactly why the second person likes the image. But given that, I have been thinking about my own response, and (unfortunately) it becomes a long list of elements. I translate "a good photograph" as one that makes me want to come back to it time and again, and here are potential reasons:

- Subject matter: There are few images of Anasazi sites in the Southwest, or Ponting's photographs from the early polar expeditions, which don't absorb my interest. IOW, most of us have subjects that fascinate us, and as long as the image is in focus and reasonably well printed, we will return to those images many times.
- Humor or optical illusion: I return to images which make me smile; examples would be many by Elliot Erwitt, Lee Friedlander, and others who have an eye for the odd juxtaposition, or many photographs by our own Austin Granger which play with either irony or optics. I would also include in this category surrealism, of which our own Alex Timmermans has posted wonderful examples.
- The "Unusual:" Here I think of the portraits by Diane Arbus, or in a darker vein, Avedon's "The West."
- Technique: Here I don't mean focus planes or printing, but older techniques such as Wet Plate, Aero Ektar lenses, or Petzvals, sort of technical elements which make an image "different." For me at least, an image which is "OK" often rises a few notches because it has a unique look which cannot be achieved without particular processes or equipment. Platinum prints also fall into this group.
- Composition: This is rarely sufficient in itself, but if we exclude portraits, I am drawn to clean geometries, such as in the work of the (once) "New Topographers" such as Robert Adams, Ray Conniff.

Now that I have listed a number of elements which by themselves, or in combination, make up what I consider "a good photo," it is also so extensive as to serve as a proof that there is no definition that really works.