That's a good question, and a hard one to answer. It's like answering the question "how do we judge what is good poetry?" Or music, or ...
Ultimately it comes down this:
the work produces a strong reaction in most people.
In the photograph above I can see the following elements which cause this
- There is symmetry, in posture (all of their arms are at their hips), expression (tired and pissed) and number (two against two). Even better it's not perfect symmetry as it's three facing one, broken symmetry is often stronger than perfect symmetry.
- Instead of using a 'rule-of-thirds' she correctly (for this shot) centered the group. Centering gives more solidity and strength, and complements the inherent symmetry. But notice they're not perfectly centered, they're off just a bit which gives some tension or energy (appropriate since the group is upset).
- They are centered in the street which seemingly recedes to infinity.
- The subjects are unconscious of her, or seemingly pissed at her which adds to the tension.
- The gestures are especially pleasing (the woman on the left and the kids in particular)
- Technically (exposure, framing, etc) the photograph is done well
A possible down side is the pole on the right hand side of the frame. I'm not sure what to make of that, one idea is that "anything which doesn't add to the picture subtracts from the picture. What is the pole doing? Nothing much other than placing them by the curb. It's chopped off,
maybe it plants them there a little more? Maybe not.
If I was developing I'd try cropping that out and seeing how it worked, but I think your stuck because of the proximity and the bag. In a way it does kind of work because it closes up the right side of the image, while the left side shows off the receding building.
Two other possible down side is the pole in the background (it competes with the woman on the left) and the car by the girls head. I might consider photoshopping those out. Of course photographs that are too perfect aren't necessarily good either, they're considered "photoshopped".
Contrasts is another principle to use in photography, here we have the contrast of closed off right, open and receding left, and the fat versus thin. Plus the woman on the right is facing back (while luckily looking toward the camera) and everybody else is facing front, more symmetry and contrast.