The rocks are looking a bit smooth to me and you might consider making them appear to have a rough texture by scattering the light reflections a bit around the edges of the harsh reflections. Personally the attention to the wood grain texture almost demands equal attention to the rock texture. Start thinking about objects in terms of a surface texture and you might really start making new art. I would assume that the roof is painted tin but I really cannot tell from the way the light reflects. Remember the rougher the texture the more midtones you need. Dithering would count as another midtone if you want to stay in low color palettes.

 

This is because light rays scatter more off rough surfaces making more midtones in your shading. Your glass windows are going to probably be very shiny if they are clean and so you would have less midtones to describe them and there is going to be higher contrast between midtones. Right now everything sort has the feel of a smooth absorbant rubber material. I hope I have helped a bit.