First of all, get rid of that nasty vignette
Secondly, learn how to do basic lighting.
The grass looks very dark while the rocks are really light. It's like the rocks are in the daylight, and the grass is in the shade, or really wet.
Thirdly, your grass, and trees very obviously "repeat" and don't look organic. Use more variety in your grass, and the make the trees not look like they are the same model. Maybe don't even use the same model.
As someone else mentioned, The fog looks really bad too.
It needs to not be there, or at least needs to start happening at a greater distance.
Use reference photos, and just compare your work to the reference. You need to take the time to notice details, and see how you can mimic that effect in your scenes.