Nevada Wier, one of my favorite photographers and, for me, a mentor whether she knows it or not, was interviewed on the website photoshop.com. http://www.photoshop.com/spotlights/nevada-wier. I love the question and response as follows:
Photoshop.com: How do you know that a photo is really good?
Nevada Wier: This is a tough question. First of all, everyone has their own aesthetic and others may not share it. I am always challenging myself to reach a higher level of visual acuity that is my own vision. In the workshops I teach in Santa Fe, I tell my students that there are four possibilities in a color image: great color, great light, great action (whether is it grand or subtle), or great pattern or composition. There have to be at least two of those. And, the gesture must be complete if it is essential to the image. However, fundamentally, I am striving for an image that has an emotional impact. I know it when I see it, because I feel it.