Mastering TG2 or any other CG tool means using it in an effective and clever way. Some people take one texture and a single surface layer on top of a boring terrain and the result will blow you away. Avoid working on things the eye doesn't see. Focus on doing the important things right. Know the common tricks that make your life easier. Fool the viewer as best as you can.
Leaving things to the imagination can be a nice trick cause the brain will fill those gaps, to a certain extent, with plausible information. You basically let the brain do part of the texturing. (This is why the thumbnails often look more realistic than the image.)
So, whatever works and looks realistic is ok.*
One thing that can help with creating realistic images is knowing that procedural textures are a great tool but worlds that are made of a single pattern repeating over and over again are boring.
The places were the pattern breaks - this is what you want to see, this is what catches our attention. This is where we start to imagine why the pattern breaks here, what is so special about this place. It's obviously special in some way otherwise it would be part of a pattern, no?
Similarly clean materials make your render look like CG. Every object is exposed to the effects of time. An old log cabin standing on a mountain can't look like it was made from perfectly straight and bright clean wooden cylinders just yesterday. It was built ages ago from logs that grew naturally, all have a different shape and a weather-beaten surface, dirty with moss growing here and there.
Fire up photoshop, open the texture and reduce saturation, add dirt, exchange textures altogether if necessary. I often create my own textures for existing objects.
The real problems starts when you try to create realistic images picturing unrealistic places. I'm thinking of stuff like a fantastic world like middle earth, a distant planet, landscapes that don't exist on earth, things that can't exist. Unfortunately, the labels "fantasy", "surreal", "alien" and the like are frequently used to just cover up images that are just bad.
An important thing concerning such non-realistic images: The most interesting images are those that are different from the world as we know it, but still look like they could exist so the viewer starts wondering what it'd be like to live in this strange world. In the best case, your image will also transport emotions.
All this isn't possible if the viewer finds himself in a scene that, for example, consists only of an untextured terrain and a sunset in front of a lake. Such worlds are empty. Imagination tells us that the rest of the planet will probably look equally boring as the scene seen in the image. We find ourself in a boring planet that looks the same everywhere.
* But you shouldn't expect praise for your work when your tricks are obvious and don't lead to an innovative image. Yes, Peak-above-clouds-at-dawn people, I'm looking at you! However, reality shows us that you can get a lot of "Excellent!"s for even a very generic image.
