On the micro-scale, the first step consists in analyzing the game mechanics, and brainstorming all ways to turn each single of them into interesting situations on pen and paper: Level Design patterns. Using Rational Game Design helps creating a wide variety of patterns, and defining the challenge represented by each of them.

On a macro-scale, Level Design requires to visualize the experience as a whole: theme, landmarks... but also the players' different playstyles, and the emotions we want them to experience. It's all about spaces, lines of sight and instincts. This great article from Christopher W. Totten has always been one of my main references in this field.

Portal is the best example that comes to my mind. The whole game revolves around the usage of a simple tool through many different patterns

Survival Horror games like Resident Evil 2 overwhelmingly play on claustrophobia thanks to narrow spaces that restrict the movement of player characters

Designing levels with unique identities is the best way to create an experience players will remember. This is the reason why I like to brainstorm with Artists the kinds of environments we could create, and the elements that would compose it.

Firstly, each level should be based on a theme strong enough for players to easily refer to: "you can find this item the volcano level", or "I finally beat the boss in the Swamp". Landmarks are crucial to ensure players never get lost, and always have a tool to estimate scales and distances. Each of them must be as unique as the level theme, and as much visible as possible.

Secondly, the greater the contrast of colors is, the more unique the levels look like. Some Designers believe color palettes convey different emotions. I partially agree with them. The truth is the meaning is highly subjective to the cultural background of each individual, and we can only assume specific colors would convey specific emotions.