One thing I think is ultimately important in an illustration is the lighting. It creates your atmosphere, feeling, sense of space for the piece. It is currently something I am focusing on. Especially pushing colors to not be exactly what is seen.
Here is a color comp of a piece I am making for a friend. I enjoy this as it, I love loose painting. I still need to learn how to let paint free. This is a step for me, it is the first illustration I have done where I used just one color for a background of creatures.