without going into the code itself I can say,
the highlights aren't light enough, the shades aren't dark enough.
this is a good case of where gamma or colour correction can be used.
the trees texture is too dark in comparison to the ground (hence a lot of the artificial look)
its very important that the artist keeps highlights, midtones and shadows consistent throughout the textures (which is also why graphical artists for large 3D projects need to be flexible)

Techniques worth looking into for more accurate lighting,
Global illumination / indirect illumination (it has many other names but these are very common)
Ambient Occlusion (this is an approximation of global illumination very commonly used, the best version I've seen so far is Screen Spaced Directional Occlusion)
Gamma Correction / Colour correction (not much experience with this myself, but its fairly common to my understanding)
shadow casting (very important in getting rid of the artificial look on large open-ish landscapes such as this one)

But I'm sticking by the lack of depth in the highlights and shadows and the inconsistency of textures. They seem to be your problems, everything else I listed should be icing on the cake.

My tips (most of them has been already said, but anyway):
- Shadows add a lot to the feeling.
- Ambient for outdoor scenes is much better with 2 colors (sky / ground) or at least one color but intensity depending on the normal.y value (more light for top facing polygons, less for down facing).
- Night scene would maybe look better if you desaturated (is that the right term?) the colors a bit - I mean to make it a little bit black&while (shades of gray). Plus slight bluish tone (you already have that though).
- Large areas of the terrain have the same color, maybe add another texture with some noise (large scale noise)?