First off, what makes a "good photo" is something ultimately subjective, and its hard to say exactly. There are some guidelines that you can follow help you determine what a good photo are:

A good photo:

• Makes effective use of light
  • Photography is the art of "drawing with light"
    • Photos: Greek for light
    • Graphia: Greek for drawing
  • Flat, total illumination tends to be rather boring
  • Plays of shadow and light, warm or cool colors, alternative tints, etc. can make a photo better
• Is properly composed
  • Shoot just a mountain solo, and even in good lighting, it might just be boring
  • Shoot a mountain in front of a lake during sunset, with a the husk of an ancient tree silhouetted in the foreground, and you have something considerably more interesting
• Subtracts the unnecessary and enhances the key subject
  • DOF it is a powerful tool of simplification and can help you compose your shots and isolate your subject
  • Longer exposures can blur elements with motion, flattening and simplifying them, enhancing the more important aspects of an image
• Is properly focused and appropriately sharp
  • Lack of focus deemphasizes the subject of your shot
  • Too much sharpness can hamper the viewers eye, overemphasizing details
• Makes effective use of color, or the lack thereof
  • Color reflects the world we see and can be a powerful tool to show vision and tell stories
  • Black & White can bring forth and enhance aspects of an image that just can't be seen when color is present
  • Monochromatic and split-tone images other than B&W can bring an artistic element to an otherwise boring image
  • Removal of color can satisfy the "simplification" rule when nothing else can
• Tells a story or depicts a artistic vision
  • A photograph thats just as photograph may be interesting
  • A photograph that tells a story has more substance