Believability. This is the art of presenting something that could potentially exist in the real world, even though its fiction. I'm not talking about realism in here, although it helps. What I'm talking about is making things look, feel and act as you would expect it to, despite the fact it doesnt exist in the real world. This is useful because it helps for a player to grasp ideas more quickly and immerse in game universe much more comfortably, and thus, enjoy the game better and learn it quicker.
You can learn this skill this by analysing and researching anything and everything from real life to other works of art. Although when you look at art made by others, do not steal their work, instead, take inspirations from it and combine it with your own ideas.
You can use the internet, books, movies, artwork and nature as sources of information to figure out how something looks like or how something works. All the information you gather will help you design anything you need that will seem believable. Creativity is useful here.
Things you can design depend on what you need to create. If you're need a monster, go study animals, humans, machines and the traits you want the monster to have. You can make the sea monster which is tough and agressive, and in turn, this should give you a better idea how to design it. Toughness can relate to the large size of the monster and agressiviness can be implied by large intimidating spikes and teeth. The fact it lives in the sea can be implied by its blue color of the skin or perhaps some fish fins and octopus tentacles. When you show this monster to a player he will be more likely to figure out what kind of monster it is and what to think of it based on its features and design.
Or how about a scout vehicle? Make it small, fast and maneuverable. Use anything means and features to imply that this vehicle is suited for its job as a scout. Do it right, and the player will be able to distinguish what he can use the vehicle for.
The thing here is to think about purpose when you design something. When designing something, ask yourself these questions: what is it? what is for? how does it work?
Why do it this way? Because its easier for a human mind to relate to things when they resemble things from real life or have defining traits that you can figure out by simple logic.
Imagine this: Two balls. Both are the same size. No differences. They can act virtually anyway the moment you first see them. Well how about this: we make the first ball look like its made out of rock and the second ball is made out of feathers.
See what happens? You can instantly get the picture on how they would behave in a game now that they are more clearly defined.
Use the same logic for characters, items, worlds and other elements to communicate to the player how things might work and behave. This will go a long way of making players get inside a game much more effectively as they can relate to it better.
(This is a part of the "what makes a good game" series)