The structure of an experience consists of seven layers -- categories or guidelines. It really doesn't matter what we call them. The following bullet points contain examples of what each layer comprises.
Context (everything perceptible)
It's even more important to understand that one experience can be made of multiple and smaller experiences, or be part of a bigger experience -- just like a game is made of levels, and those are made of gameplay sequences, and those are made of gameplay, and that is made of actions, etc. It grows infinitely big and infinitely small.
It's the ancient principle of correspondence, the theory of relativity, the phenomenon of fractals, or the relationship between the microcosm and macrocosm. That's also where it easily gets confusing to analyze the different layers, because things move up or down the scale, and everything is relative.
Our seven principles can be considered as seven linear steps that can put the player into a greater state of engagement. If the player dislikes one step, it can be enough to prevent her from continuing the experience. Ideally, we want the player to be seduced by all the steps as much as we possibly can. Simply put, all steps should be better than what can be found in past games, films, books, or music.
A greater degree of fun is experienced when we simply experience something of greater quality. Basically, engagement allows the potential fun of a game to emerge.
Still, all the products have their own strengths. Some games will offer greater visuals and others greater mechanism or emotions.
In order to produce greater states of engagement, to do less is more, because the more we add details, the more we increase our chance to make a mistake or to create things that are disruptive or disturbing to the experience.
More importantly, everything created must have a compelling reason to exist, and ideally it should empower the rest. It's pointless to add 50 accessories to a character if none of them actually tells us more about him. The simple color of the cape, the shape, or the way it moves should alone tell a lot.
It's always great to keep in mind that time is money.
A game isn't supposed to be fun for everyone; it was designed for a very specific audience. The group we can attract is gamers. Is everyone a gamer? Absolutely not.
Fun is subjective, but not entirely. From one perspective, because we all experience different things and are born with a certain approach to life, we might find things fun that wouldn't necessarily be so for others. Still, experiences forge our judgement; those experiences are limited to what exists. That means if you understand what exists, you can obviously make something that's more fun for your audience.
That means fun is only wholly subjective if we don't know what is out there, or if we can't create something more fun than what the audience can create in its own mind.