Hi Mander,
I think you are off to a good start, but there are few things that I see wrong with these pics.
1. I agree that the majority of these are underexposed and the flash ones are a bit harsh. What kind of lens/lenses are you shooting with?
2. Watch composition. In #1 and #2 there are trees coming out of their heads. Also, the last one would have benefitted from a much closer crop.
3. A few of them, especially #3, have color casts. The third looks exteremely blue, so remember to set your white balance properly.
I think my favorite of the set is #4.