An emergent behavior is a consequence of the rules. Take the rules of, say, chess: the rules of chess do not explicitly refer to the concept of initiative or that opposite colored bishops tend to be drawish. But these and many other characteristics of the game are determined by the rules. We see emergent behavior in many complex physical systems (fluid mechanics in physics, for instance) -- or more to the point, we see it in the material happenings of any complex game world.
The various characteristics of explosive barrels in Doom is one canonical example. The rules which govern their behavior are very simple; nowhere does the program say anything about how they can be lined up for a chain reaction.
Once barrels started exploding in chain reaction, the virtual world had suddenly become robust, palpable, realistic. This was an amazing moment, but very little of the player's energy was invested in playing with the system -- yet. Doom struck a highly linear and simple tone, for it did nothing to encourage the player to experiment with the scenario.
From a certain point of view, Doom could be considered a sandbox: we remove the "EXIT" and the player wanders around killing baddies, doing as he likes. From the same point of view -- and this bears especially on how we commonly use the term nowadays -- the production of a "sandbox" game is a subtractive operation: subtract the missions, the main campaign, the narrative or whatever formatively binds the game's progression, and you have a "sandbox." The player can fool around without doing anything "on task" or so.
This is the sandbox we mean when we speak of "Sandbox Mode" (as opposed to "Campaign Mode"), and it is closely similar to how the term is used in software development.
In general terms, if one removed the objectives of a game to produce unguided play, or lack of narrative, one would makes a sandbox in some subtractive sense -- but not in a productive sense. True sandbox design means adding game behaviors which, in combination, produce interesting emergent behavior, but it also means adding some reward for free play. Emergence is good, but a free-play oriented framework is also necessary.
While meta-play and multiplayer are certainly two entirely different phenomena, they have some things in common and they often happen simultaneously, so we might consider them loosely together.
Meta-play normally means a different approach to playing, where the player is no longer playing the game as it was designed, but messing around with it and doing amusing things. This includes exploring glitches, testing the game's limits, creating and pursuing personal objectives, and other things which were not necessarily intended by the game's designers. "Hmm, I wonder how many resource harvesters I could build..." or "How far can I drive off the track, and what happens then?" or "Can I finish the game without getting the spider ball?" -- this kind of play.
This relates back to our opening discussion of adventure games, whose design tends to be in the form of lock-and-key puzzles. One implicit challenge in such games, and one way by which mastery can be measured, is in figuring out the shortest route.
When the game is played in this attitude, the metaphor of adventure falls away, and the player instead thinks consciously of the underlying system, how to optimize given the rules of the system -- and even how to break the rules of the system. Though it operates on a different level, sequence-breaking is very sandboxy and very meta, and lock-and-key style adventure design encourages it, from Super Metroid (1994) to Switchball (2007).
The key here is that the game might support sandbox-style playfulness or meta-play, whether or not it was designed to do so. Sandbox is a much wider genre in terms of play than it is in terms of explicit design: a wide variety of games can be played in a sandbox style -- it just depends on the ingenuity and creativity of the player.
Even chess can be considered a sandbox game, if you look at it in the right way. It need not even be played as a competition: instead, you and your opponent could cooperatively explore the potentialities of the game, to see how certain interesting structures can emerge -- to "meta-play" the game, not competitively, but critically, analytically, imaginatively. (One could well argue that if you look at chess in the right way, meta-play happens quite frequently over the course of a normal competitive game.)
Indeed, any sufficiently complex game can be considered a sandbox if one of the aims of the players is to explore the implications of the game's rules. The metaphor of "game world" becomes strained, but it is possible to liken the space of potentiality opened by the rules to a game world, which the players can freely explore.
The point is that it does not take two opponents to play chess; instead, one can play in a creative way -- solving the eight queens problem, for instance, or producing an elegant endgame. The traditional card game solitaire is not really a sandbox game; but a solitary game of chess can be. The interesting point here is that there is a space of free-play potential even before the opponent enters the scene.
The case is similar with multiplayer: the game need not be specially designed to support rich sandbox gameplay; it needs no carefully-crafted narrative framework, no believable characters, and so on. By contrast, it takes only a modest arena to produce all the necessary strategic interest to support a rich multiplayer experience. Even the simplest of MUDs can do it. When it comes to multiplayer, we can strip things down quite a bit, as the opponent provides much of the necessary framework.
This is no argument against complex multiplayer worlds. World complexity often leads to more nuanced strategy, which is a good thing. But speaking minimally, all a multiplayer arena really needs is a set of rules.
Likewise, if the player approaches the game in either an ironic, analytical, or deeply-invested way, then the experience can rest on the simplest of pleasures -- such as riding horses around together in a wilderness, and looking at a randomly-generated landscape.
But on the other hand, if we subtract multi-player, or we subtract that meta- level of player interest, even the most realistic game-world can lose its interest very quickly. A realistic simulation can be a great multiplayer arena, and a great foundation for building a game-space, and it may indeed be fun to explore for a little while. But it must be recognized that realistic simulation on the one hand and gameplay/presentation on the other are very different phases in development, and if the principle challenge and purpose is not being supplied by multiplayer, then some directing framework must be supplied by the designer.
Game design itself is, undoubtedly, the ultimate sandbox game: you the designer get to determine the game's objectives, and not only that, but also create and assemble the artwork and other presentation elements, balance the game as you see fit -- create a whole world to play in.
Modding is quite similar to game design in that sense. The main difference is that modders do not write game engines and they do not design the larger framework. Their role tends to be limited to top-level design, though of course this varies from game to game.
Ten years ago, one would be wise to remark that "the future of gaming is modding." But over the course of the past decade, modding itself has become increasingly part of playing the game, and the line between playing and modding is now and forever blurred. From the simplest "scenario editors" of the late 1990s through Neverwinter Nights modding tools, to Crytek's Sandbox, game production has increasingly focused upon in enabling and encouraging player design, and today's games often present certain forms of design as a core ingredient of the gameplay.
For years, modders have been using Maya rather than Creature Creator. Spore's obvious innovation is that now every player gets to mod. Spore is not alone in this, of course. Indeed, LittleBigPlanet is arguably even more progressive in this area. The key is the creation of a game that works towards the objective of player-generated content, and designing tools to enable novice modeling and design.
We will be discussing Spore as a special case study, below. The salient point here is that while Spore makes an art of erasing the difference between modding and playing, the same thing has been done for years in a less integrated and novice-friendly manner, but far more completely.
Second Life is still another canonical example of a game that is designed to be modded. It produces an amazingly real analogy between (on the one hand) the clothing designer, construction worker, or architect/engineer, and (on the other hand) the 3D modeler. Players are sometimes even paid for their 3D models in real money, just as real-life carpenters are paid for their cabinetry work.
Whether people play perfected visions of themselves or ironical caricatures, the combination of multiplayer and modding assures a permanent place for Second Life. On the other hand, one key element is that there is no game-worthy interest. It is a pure sandbox, and so it suffers a lack of interest, from a lack of what we have been calling 'framework': a lack of direction.