Depth is my primary metric of quality for a game. I believe depth is a good metric because it is “simple” and “generic”. Unfortunately it’s not simple in the way of being simple and relatable to understand. It’s simple like GDP is simple. It’s one final number that represents a whole ton of things going on under the hood. Depth is the emergent result of a lot of different things coming together in a game. Depth, like GDP, is a generic metric in that it doesn’t care what’s being invested in, it could be medical, military, education; puzzle game, RTS, RPG, FPS, or fighting game, it only matters what the final outcome is. Depth doesn’t encompass everything about a game, the same way GDP doesn’t encompass everything about an economy, but both are fairly important metrics regardless. Unlike GDP, there are less ways to fake depth and end up with a cheap result.
I define Depth as the number of states that are differentiated from one another, balanced against each other, and currently known about/preferred by the playerbase. State is the current condition something is in at a specific time. A state with regards to games is the current condition of everything present in a game at a moment in time. Depth is the sum of these states after passing through 2 filters: redundancy, and relevancy.
We start with Possibility Space, which is every single state possible. We filter those into Absolute Depth first by removing all states that are redundant, that are just copies of one another, such as rotations or mirror images of the game board in Tic Tac Toe or Go, or more powerful but functionally identical weapons in RPGs. Then we filter Absolute Depth into Relevant Depth by removing all states that are underpowered and therefore not commonly used in play, or the ones that are unknown to the player community at a given time, such as those that use undeveloped techniques or unknown mechanics. The final result is a measure of the effective complexity of the game.
Okay, so, why the hell would the effective complexity of a game matter? What does it matter if a game is more complicated? For this, lets go back to Raph Koster’s Theory of Fun. The gist of his theory of fun is that fun is derived from winning at something inconsistently, like a coin flip. Fun is also derived from improving your consistency over time. Something you can win at effortlessly is boring, and something you never win at is frustrating (this is backed up by Flow theory too). Random things can trick the brain, which is why gambling can be fun, but most people eventually catch on and stop playing, unless they delve into superstitions about luck.
However there’s also a bit of a contradiction there, if you improve your consistency over time, then won’t something that’s fun now eventually become boring when you’re 100% consistent? That’s true. Depth gives players many different measures of consistency, so while you may be consistent at one thing, now you have something else to get consistent at.
Raph Koster’s Theory of Fun posits that fun is the joy of learning (probably because learning things makes us better at surviving, so we adapted to reward learning neurologically). A deep game has a lot to learn about. Therefore a deep game is a fun game.
On top of that, the experience of playing a deep game is different from playing a shallower one. Deep games typically have more choices, and more possible consequences for those choices, requiring more complex thought about each choice. Many board games with less board states are easily solved (connect 4, checkers), where more complex ones require more arcane heuristics in order to perform well at (Go). Simpler games are more about doing 1 thing right, where deeper games are about thinking about future consequences more. Deeper games involve more interesting decisions, as per the Sid Meier definition.
Smash Bros Melee might have less buttons and less attacks than a traditional fighting game, but you can get more results from each move than you can in a fighting game, because Smash Bros is highly responsive to the relative positions of each character, and the timing with which attacks are hit. This isn’t to say that Smash Bros is necessarily better than a Fighting Game though, because both a few nuanced moves, and many differentiated moves are equally prioritized under depth theory, as long as they shake out to the same number of relevant states.
Later Smash Bros games did a lot of work to remove a lot of the nuance in Smash Bros Melee moves, by making them less responsive to differences in timing and spacing (less sweet/sour spots, reverse hits no longer work), by reducing the effect of defensive mechanics during combos, and removing options outright. These games are comparable in their options, but have less depth. This makes progress less clear, since there are no longer an array of clear techniques and strategies to master, and requires players to work harder to get smaller rewards for their effort.