If there is one thing that has a huge effect on any render, then it must be light. Just take a look at these two renders "Sunny" and "Cloudy". The difference between those two are just one simple and powerful light source that mimics the Sun.
As effective a lot of light is, just as effective can it be with as little light as possible. Like "Drink up me heartie yo ho!"
In short. Wherever there is light, ther must be shadow. The shadow may be subtle and hard to see. Light can be the difference between a good and a bad render.