Because it has variable colorful textures and most of all, I think this work shows my texture skills well. No matter how you did modeling and texture mapping perfectly, you can't get a perfect output image if you don't use lights well. Light makes shadow and the shadow makes 'depth' in a scene. That's what we call, '3-dimensional'. I've seen a lot of 3D works where this important element is totally ignored, and those works don't look realistic.
Lighting needs a lot of experience and knowledge, and that's why it's very difficult.
The second most important process is 'texturing'. It gives 'characteristics' to each objects and they can be new,dirty or rusted or whatever. The more delicate it is, the more realistic your work can be. It can solve memorial problems such as rendering. Modelling is also a very important process.