The lighting and shading (particularly when it comes to the vegetation, wood and metal) definitely need work, I think Ivan gives really good advice. The sky is a lesser issue that you can deal with at some later time I think, since it's easier to compartmentalize that whole feature.

I will say though, for the most part the rocks and the grass look extremely good. Especially the last picture with the new shadow effects look very photorealistic, so nice job with those.