Mario 64 Game Play
Mario 64 blends (apparent) open-ended exploration with continual and clear direction along most paths. Players always have lots to do but are given a lot of choice about which parts of the world they work on and which extra stars they go for. The game also avoids a lot of the super-linear, what's-on-the-next-screen feel of side-scrolling games and gives players a sense of control. In Mario, players spend most of their time deciding what they want to do next, not trying to get unstuck, or finding something to do.
A major decision made in the design was to have multiple goals in each of the worlds. The first time players arrive in a world, they mostly explore the paths and directions available. Often the first star (typically the easiest to get in each world) has been set up to encourage players to see most of the area. So even while getting that first star, players often see things they know they will need to use in a later trip. They notice inaccessible red coins, hat boxes, strange contraptions, and so on, while they work on the early goals in a world. When they return to that world for later goals, players already know their way around and have in their heads some idea about how their goals might be achieved, since they have already visited the world and seen many of its elements.
Mario's worlds are also fairly consistent and predictable (if at times a bit odd). Players are given a small, simple set of controls, which work at all times. Simple though the controls are, they are very expressive, allowing rich interaction through simple movement and a small selection of jumping moves. The controls always work (in that you can always perform each action) and players know what to expect from them (for example, a triple jump goes a certain distance, a hip drop may defeat opponents). Power-ups are introduced slowly, and are used consistently throughout (for example, metal Mario can always walk under water).
These simple, consistent controls, coupled with the very predictable physics (accurate for a Mario world), allow players to make good guesses about what will happen should they try something. Monsters and environments increase in complexity, but new and special elements are introduced slowly and usually build on an existing interaction principle. This makes game situations very discernable — it's easy for the players to plan for action. If players see a high ledge, a monster across the way, or a chest under water, they can start thinking about how they want to approach it.
This allows players to engage in a pretty sophisticated planning process. They have been presented (usually implicitly) with knowledge of how the world works, how they can move and interact with it, and what obstacle they must overcome. Then, often subconsciously, they evolve a plan for getting to where they want to go. While playing, players make thousands of these little plans, some of which work and some of which don't. The key is that when the plan doesn't succeed, players understand why. The world is so consistent that it's immediately obvious why a plan didn't work. This chasm requires a triple jump, not a standing jump; maybe there was more ice than the player thought; maybe the monster moves just a bit too fast. But players get to make a plan, try it out, and see the results as the game reacts. And since that reaction made sense, they can, if needed, make another plan using the information learned during the first attempt.
This involves players in the game, since they have some control over what they want to do and how they want to do it. Players rarely feel cheated, or like they wanted to try something the game didn't support. By offering a very limited set of actions, but supporting them completely, the world is made real for players. No one who plays Mario complains that they want to hollow out a cave and make a fire and cook fish, but cannot. The world is very simple and consistent. If something exists in the world, you can use it.