Compare your moss picture to
this one.
Your leaf picture to
this one.
Your Child running to
this one.
and your valve picture to
this one.
1. The problem with the first one is that it's something we have all seen. In fact, in reality, almost all pictures on this site have been seen before, it's how you express your feelings/emotions that will separate your picture to others. The moss picture I linked is a GREAT example of someone seeing the same as every one else and doing it differently. Resist the urge to just take pictures of things that interest you and focus on going out with a particular goal or vision of what you would like express. Technically, the moss picture is good. Good focus, good lighting, but just not imaginative. What kind of feelings/memories do you get when you see this picture? Can you express those feelings more clearly?
2. To be concise, it's just a leaf. It's put straight into the middle of the frame, it fills less than half of the frame, and it's a little underexposed.
3. The child running was composed awkwardly. A rule of thumb is don't cut off a person's appendages at the joints. This one is cut off at the butt, and because you can't see what the child is doing you really can't know if the child is jumping, running, walking, standing? Part of what makes these types of pictures work is that you know the emotion involved with the action.
4. Personally I think the valve picture is the best because is seems like this was most calculated and conceived. However this comparison to the other valve picture might give you some inspiration. Of course this is my assessment of these our my own and totally up to for interpretation.