You should also seek to deepen your awareness in these three categories:
1. Shapes / Silhouettes
We humans perceive objects by their edges first. So for clarity of statement, you should think silhouette first, and make sure your object is identifiable by it silhouette. Pretty much like the Terminator, when you enter a room, your brain scans and identifies your surroundings (the purpose is to highlight potential threats; we are so awesome).
To increase interest, things should be clear and easily readable, not confusing. Within your art style, try to make identifiable props and characters just by looking at the silhouette. We work in 3D, so it might be difficult to give a clear identifiable silhouette from every angle. But use the gameplay camera to identify the most important angle, and give it some extra love.
Often, a slight exaggeration will help you describe an object through its silhouette and enhance its emotional charge. Try to sprinkle a bit of caricature in your shape work, it will surely be... spicier!
Don't assume this doesn't apply to you because you work in a "realistic style". You still need some amount of characterization to make art interesting. You need to distort realism, to make it louder, in order to be entertaining.
Now be aware that your shapes/lines and their directions carry an emotional charge. You need to orchestrate these shapes and lines so they can carry a unified message. Horizontal shapes and vertical shapes do not evoke the same thing. One calls for quietness, and the other calls for austerity. Make sure your shapes' arrangement supports the ongoing story you are trying to tell.
Doberman photo by IIi Ivan; bloodhound photo by Claudia Krebs. Source: Wikipedia
Please bear with me for a moment while I wear the title of Captain Obvious. These two dogs trigger different responses; one looks much more aggressive than the other. It's all because of their physical appearances. The Doberman looks more agile, faster, and aggressive because of his lean and sharp silhouette. The bloodhound feels heavier and slower, and his droopy eyes give him an air of laziness; he doesn't look very alert.
These design attributes doesn't apply only to living things. You can create anything and give it a different appeal. You can design a cheerful looking table or a sad one. The story should dictate your design choices. Remember Anton Ego, from Pixar's Ratatouille? Let's revisit his office. His personality and physical attributes are reflected in his personal space. He, himself, is austere, tall, slender, and dark. His office has a high ceiling, a narrow space, tall vertical frames and windows, dim lighting and is hushed. Even if you're working in a so-called "realistic style", give yourself some freedom to customize things according to the story.
Colors & Values
"Colors" is a lengthy subject, and a very subjective one, too. There are no hard rules; if there are any, they all have exceptions. So just keep few things in mind: Colors come with their ranges of temperature and emotion, and based on the story you want to tell, there might be colors you want to avoid. Colors can also be used to help with readability of gameplay mechanics. Explosive red barrel! Ever seen that one?
Colors can be used to create symbolic links. This can be subtle but yet powerful, as in the intro sequence of Pixar's Up. The artists used the fuchsia color as an expression for Ellie, often seen on objects or clothing she uses -- and finally use that established symbol to illustrate her passing away when the pink sunlight disappears in the reflection of the window. Beautiful!
There are multiple books talking specifically about colors, but an efficient way to study colors is to watch movies and dissect the color choices made, and their purpose in the story. We're not only concerned with harmonious color combinations, but story mood and harmonious color combination.
Composition / Leading the Eye
Composition is probably the most elusive art concept. If I can reduce it to one simple statement, The art of composition is the art of directing the eye through a picture. Let just assume there is no bad compositions, only misused compositions -- stronger or lesser compositions. What may work in a specific story might not work in a different setup. Your sole goal with composition is how you want the player to read and understand the space/story that is being proposed.
The most common tool is to use conflict and contrast, and directional lines or shapes. Conflict in shape, conflict in color and value. The eye will usually jump on the highest contrast within a frame. When you have your point of focus, make sure the other elements don't fight too much for attention. At best, the hierarchical organization of your elements should altogether gradually lead to the focal point. Composition is often misunderstood, and reduced to the tier rule; there is much more to it.
Function, Function, and Function...
When trying to build a world, you want people to buy in, and forget they are within a fiction. You want to distract them away from the backstage. You don't want them to see the set; you want them to feel the world as naturally as possible. The props that will populate your world should feel believable and functional. It's a no-brainer. Your objects and scenes should be as descriptive as possible.
Too often I've seen artists struggling trying to embellish something by adding random details. They feel something is not quite right; they are trying hard to fix it, but unfortunately from an angle that isn't broken. With them, I will sit down and evaluate the piece from three specific angles.
Function in design. Does the object appear functional? Would I be able to use it if it was right next to me? How do I carry it, how do I turn it on, how do I access it? Can my hand fit on the handles? (Artists tend to make them either uber tiny, or friggin' huge.) What details can I add (or remove) to make it purpose or usability clearer?
Function in gameplay. In the desired experience, how important is that prop or space? What is its role in the whole level design scheme? How does the player interact with it? How can we help the player understand the desired interaction? Does it attract too much (or not enough) attention?
Function in story. Does this props or space sell the fiction well? Does it contradict anything? What story-related details can I add to add visual interest? Does it thematically blend well enough with its surroundings?
When you start adding details, remember that it's not because you can, but that it's because you should. We work under multiple types of constraints: textures, polygons, time. So you must select and prioritize what best characterizes your props, and work hierarchically from the most obvious "can't do without" to the lesser details.
Make sure what you add bring value to the game for the player, and make sure it doesn't conflict with your overall composition. It's best to evaluate all this from the player perspective, with as much of the other assets integrated as possible -- not the in Maya shaded view.
Here is a short example of details and story. While you were modeling and texturing the most beautiful door ever -- the second most popular prop after barrels -- your AD drops by and asks you to age it a bit. It feels too brand new. The reflex of many artist at that point is to jump into Photoshop, add an even layer of dirt on the entire diffuse and specular texture, and voila!
Well... let's push things further a bit. Let's work from logical "a cause and effect" point of view. By the way a door is used daily, there will be multiple distinct types of traces.
The pink area, is where the hands are more likely to touch the door, so over time, sweaty palms and greasy fingers will leave a thin layer of dirt. The yellow highlighted area is where the feet or objects (bicycle wheels and pedals, maybe) might collide with the door, leaving dents and scratches. It's an area also prone to water damage.
The green area will be stressed by the incessant opening/closing of the door, constantly hitting the frame, it will likely chip the paint off and wear the edge out more than the interior edges. When adding details, try to use them to tell believable micro-stories. The more believable they are, the more the player will buy in.