The greens look off, try editing them to be less intense. All of your shots have excelent composition.
#1: Nice shot, the tree kind of ruins it though...if you zoomed more on the squirrel or cropped so it was just the squirrel it would definatly help. (if you dont have a bigger zoom, think about getting one)
#2 could use some increase in shadows and contrast.
#3 Try turning off the flash or diffuse it. basicly just Increase the shadows (if you were indoors you could bounce it off the ceileing (if you have an external flash) dont try bouncing the flash off the sky, it doesnt work to well

) Oh, and use a red eye fix on the squirrel.
#4 Perfect other than the grass. Maybe its the normal color of the grass, it just doesnt look right to me
#5 Remove the squirrels red eye
#6 If the person wasnt wearing a jersey (if he had just some jeans and a regular ol t shirt or a suit or something youd wear on a hike(something basic and simple)) and didnt have the stroller it would be something id put on my wall. It would also be worthy of my wall if the person wasnt there at all. (I personaly dont like people in my photographs)
Alll these photos are pretty good. You obviously have an eye for photos or know of the rule of thirds (if not google: Rule of Thirds)
If you want to continue with photography, you are definatly going to want to invest in another lens. Like an 80-200mm or 70-300mm. Something with more zoom than you kit lens. 200-300mm is good enough for anything youll be shooting in the near future. Since you like shooting people, a 35-100 or something close to that is a good choice.
Or you can get a 2x Teleconverter which doubles your lenses focal length. (so youll get a 36-110mm lens) A teleconverter is a good thing to have. It can save you quite a bit if you dont want to buy a new lens.