I'm partial to games of exploration. The Ultimas and their relatives were my early favorites. These days, I play games like Grand Theft Auto, Godfather, Spider-Man 2 and Oblivion.
Why? It's the joy of discovery - the pleasure of finding things out. So what makes a great open world game? In the context of video games, an open world is a toy upon which a game can be built. So first, what makes a great open world?
Big, not too big
It's no secret that we measure the size of our universe in the time it takes to cross it. The world shrinks with the inventions of the railroad, the automobile and the airplane. Unlike other open world games, the world of Superman Returns seems in turns enormous and tiny, depending on whether you are walking or flying at supersonic speeds. When a world is too big, we can spend all of our time simply getting around. Too small and it seems cramped and unimaginitive.
There must be some novelty to every distinct area or region. The 1987 game The Faery Tale sported a world of over 17,000 screens. There were so many repeating patterns of landscape and it took so long to cross that it was hard to stay interested in exploring the world. In an interview with Game Informer, Radical, in creating Hulk: Ultimate Destruction, specifically addressed this issue of variety.
The mechanics of a world are crucial to its experience. They are the laws of the game's universe. It is important that the game itself cannot subvert these rules. In GTA, this includes the way cars move collide and interact, that any car can be rigged with bombs and the general behavior of the people and objects that inhabit Liberty City. Another important mechanic in GTA is police response to player actions. The world of GTA is rich with mechanics largely orthogonal in purpose. It is this richness that makes GTA great. In Spider-Man 2, the central mechanic is web-swinging. The world of Spider-Man 2 is designed to be experienced through web-swinging and so it is an inseparable element of Treyarch's Manhattan. In GTA, Spider-Man 2 and Superman Returns the speed at which one is able to move through the city is variable and its 8control is in some mechanic anchored in the game world itself. This provides a sense of enormous freedom. In some of these games, this speed increases as the player progresses through the game.
Even so, at this level, the open world is still a toy. Now let's ask the question, what makes a great open world game?
The Childhood Game of Tag
Of course, GTA 3 is the prototypical open world game. Recently, I started playing it yet again and it still may be the best. What makes it fun? The underlying element is tag. The ancient children's game is incredibly simple, infinitely flexible and has strategic depth. Tag benefits from richness and complexity in a large world. The ideal tag world has nooks and crannies in which to hide from predators and from which to ambush prey. Layered on top of that, in GTA, there a number of other game mechanics, many - most? - of which are themselves variations on tag.
These layers of tag and alternating roles of hunter and hunted in GTA 3 stand in contrast to Spider-Man 2's races which also aim to capitalize on a large in-game world. Unfortunately, racing is much less fun than tag. Even though it's one of my favorite games, Spider-Man's races don't appeal to me. What appeals to me about Spider-Man 2 is the absolute euphoria of swinging and the crazy missions (levels?) like Mysterio's burning theater and the Statue of Liberty.
Tag (from Wikipedia)
Rules complexity: Low
Strategy depth: High
Random chance: Low
Skills required: Running, Hiding, Observation