IMHO realsim is achieved by a number of factors:

1. textures must be believable.  We have all seen realistic textures but they are the wrong color, too large, too small or don't fit the object they are applied to or fit with the overall scene.  A perfect example of this is often times when stone or marble buildings are built, the stone is quarried from deposits that exist locally.  There are exceptions to this of course, but a bright blue marble castle in the middle of a mountain scene with white chalky cliffs just doesn't work.  Nor does a bright red new brick material applied to a castle work.  The eye expects to see old rugged textures on objects of this sort and the artist must allow him/herself to be aware of this.

2.composition doesn't make a scene realistic, just more elegant and appealing to the eye.

3. diversity.  We have all seen images of trees or plants that look like they were rubber stamped throughout the image - there is no variation to them.  Realism is achieved by the diverse and random manner that nature itself takes.

4. details.  When constructing a scene one needs to consider what would exist there in real life.  If you are creating a river scene, then two sloping terrains with a river running through it isn't very conviencing.  Rocks and boulders along the waters edge, a broken tree trunk lying in the water, plant life growing along the shore and draping in the water,  maybe a broken down shack or house visible in the distance, a bird or two in the distance - or maybe a whole flock of em.  The details can be few or lots, but some must exist in the scene in order for it to be believable.  We have all seen various images of nature and as such we all have come to expect to see certain things in various environmental settings.  A cactus growing in a pine forest is just as taboo as a lush green fern growing in the middle of the Sahara desert.  The artist must strongly consider what naturally appears in various natural environments and then include them in their scene.  The details, while they may be small are essential in order to transport the viewer into the scene and make it believable.

5.  variety.  I hate seeing forest scenes of only ONE darn tree genus.  While there might actually be some forests containing only ONE specific tree, that is not the norm and therefore the average viewer wont find it believable.  Variety however will be more eagerly accepted by the eye as that is what is most frequently seen in the would around us.

6. flexibility.  I cannot count the number of times I have a really great model with cool textures and want to create a scene around it.  But as I get working I realize that either the model doesn't fit with the surroundings, or the surroundings don't fit with the model.  I try and try and try to make the two fit together but often fail.  The thing that works most often for me is to keep the surrounding environment and replace the model with one that fits more naturally.  I ultimately end up with an image completely different from what I initially wanted, but I try to allow myself to be open to change and not force a round peg in a square hole.  It is depressing that I cannot make all things work together in my images, but flexibility is the key when you can't do something and it must be applied when you run into composition problems.   Otherwise all of my images would just be shiny spheres hovering over water ;^)

I have a book at home that is a pictoral layout of numerous buildings in Italy.  It focuses on the interior as well as exterior of buildings and their decor.  The book emphasises the ideas that go into the style of much of Italy's architecture in that wood, stone and earth are all used and their corresponding colors all are various hues of their natural counterpart and local surroundings.  The pastel yellows on buildings reflect the often arid landscape, the peaches and reds mimic the red earth tones etc.  There is a lot to consider in order to create a realistic image but picture books are a huge aid in struggling through this hurdle as well as viewing the artist images in the photography gallery.

Hope this helps someone - 3d artistry is a difficult hobby, but if we didn't love it so much, we wouldn't keep working at it. 

G&C