Plex Level 1
For example: Sentences. Everyday you and I design original, creative sentences without even thinking about it. Even if we have completely forgotten our secondary school grammar lessons, we all know subconsciously that there are certain acceptable (grammatical) patterns sentences can have, and we all know roughly a quarter of a million details (words) and which slots (parts of speech) each can go into, and which transformation is required to make it fit in a different slot (e.g. friend, friendship, friendly, frendlier, frendliest, make friends, befriend, friendless, etc.) Designing sentences is simple because the patterns we are dealing with are one-dimensional, they are only plex level 1. Specifically they exist in the dimension of time, which when I type them for you is represented visually by the X, or horizontal, spatial dimension.
Plex Level 2
Clothing, or monsters (appearance only), or monster AI, or characters (appearance only), or character personalities (at one point in time only), and many of the other elements of game design usually labeled "Content" are all of plex level 2, because you have to be able to hold in your head 2 dimensions of design at once. These are not always the same two dimensions: in the case of clothing one is shape and one is color/texture, while in the case of AI one is time and one is situation.
Plex Level 3
Some kinds of design are more complicated still. Music, for example, must be designed in the dimensions of time, pitch/timbre, and interval/harmony. There is usually a term for the intersection of any two of these dimensions. For example, time x pitch = progressions, which add up to melody. Each instrument (assuming that these are single-note instruments like flutes, not chord instruments like pianos) then plays its own melody/part, adding the dimension of interval to give the music depth/orchestration. The finished piece of music is thus a journey through time where at any given instant several instruments are having a discussion, speaking different 'words' in different 'voices'.
A story too is a journey through time. The voices are characters, naturally. A chord would be a trope, a progression would be a sub-unit of plot, interval would be character dynamic, timbre would be atmosphere, a melody would be a single character's plot line, and the finished, orchestrated piece of music would be the story.
Animation, too, is of this class - we take our previously described 2-dimensional visual elements and add the element of time (and relative motion of course, since that's what they're doing over the span of time).
I believe gameplay is also plex level 3; its dimensions would be time and situation, like in monster AI, plus the player's strategic/aesthetic choice among the options available at any given time.
Plex Level 4 (dun dun dun...)
And finally we have come to the 4th and most difficult plex level - putting the music, story and animation together to get a movie or a game (games are basically movies plus the additional element of gameplay). Being a game designer is a huge ambition because you have to balance all these elements against each other to create a product that satisfies your audience.
Definition of 'satisfies your audience' from a post of mine:
Entertainment is all about manipulating people's mind and emotions in various ways, and when a story pushes all the right buttons with the right timing and intensity, the audience is satisfied and will clamor for more.
So what does it take to satisfy an audience? What are they searching for in our stories? It's not just about showing them something new and surprising to alleviate their boredom, although that's important. You gotta catharsize their emotions - make them laugh and worry, be fearful and amazed, go 'awww' with pity and coo at cuteness, be angry at injustice and righteously satisfied when it get fixed, you gotta stroke their egos, make them feel better about themselves and their lives, and so on.
And as if that isn't hard enough, they also want to feel like they learned something, so you have to put in some interesting exposition and probably also some insightful philosophizing about people, life, emotions, morals, and that sort of thing. But of course you can't be preachy, audiences hate that.
On top of all that you have people looking to you for wish fullfillment and escapism, to "fulfill desires no more than partly satisfied by life" and "experience an imaginary world more lifelike than life itself, more directly and honestly concerned with essential problems, more supple in its expression of every aspect of man's nature, less burdened by distracting irrelevancies". They aren't just looking for a better place to go and more interesting situation to be in, they're looking for someone to be - material that they can use to build their personal mythologies and identities.
And finally many people are lonely and are searching for soulbrothers and fantasy romantic objects among your charaters, so if you want to satisfy that segment of your audience you need to provide vivid, deep, and appealing characters for your audience to identify with and lust over.
And that's just what's satisfying for story; a game designer has to worry about what's satisfying for music, animation, and gameplay too, not to mention the balance between these.
So... are you sure you want to try to design a game? If you're really sure, keep reading, because next time I'm going to start talking about how to design a whole game. (But I'm not writing any more unless I get some comments! :( )