Lighting.
Lighting.
aaaaannnndd then lighting.
That's the one area that I am trying to focus on. It took me a while to understand why, with all the preset materials that were physically accurate, complicated, and tweaked to perfection as to why my scenes weren't looking as good as the stuff I was seeing coming from others. So, I dived into materials and got a solid grasp on those, but now I know that honestly, its more lighting than materials. You should be able to have a simple diffuse material look really good, but to do that you need to know what you are trying to do with the lighting. Will gave me the task of picking a model, texture it with a flat diffuse texture, and then light it 5 different ways with custom made HDRI's to give it 5 totally different looks.
That is where I am working on now, and that is where I see a lot of effort going into these awesome realistic scenes is in the lighting.