Apart from that which has already been mentioned, one thing that strikes me a lot when I see a scene, and this is something I always notice, is that some elements (objects) in a scene can look stiff. 

It's not easy getting that certain 'heaviness' that is apparent in the real world. I notice this more on poser models that end up looking like shop window dummies instead of people, but this works in the same principle in landscapes.

Things to look for to get a more realistic 'heaviness'....

Is the object, a rock, a boat, a tree etc. actually resting on the ground, terrain or water... or infact any other object? Do added objects sink into the ground if the ground texture is to give to representaion that it's soft like sand, mud or snow for instance? Take a look at a similar object in reality, look how heavy it appears. Sometimes just 'landing' objects is not enough, you may need to lower them a little more.

*Is the object, stood bolt upright? In real life it can do but if it's slightly leaning in one direction, it looks more natural.

Another thing that can drastically alter the appearance of objects is light and shadows. A well placed shadow can give the impression that it actually is as heavy as it would be in reality.* 

Obviously if one isn't trying to make a scene look realistic then please disregard the above.