When it comes down to it, games are about meaningful decisions. Providing the player with the opportunity to make decisions or choices is the key to good design. Of course, lots of other factors needs considering, as long as they don't distract you from the main issue – the provision of choice to the player(s).
For a decision to be meaningful, there has to be a trade off between risk and reward. What is the player putting on the line for what kind of reward? Is it a real choice, can the player take different approaches either with different risks associated or have different reward outcomes.
For a decision to be made, there has to be time to consider the options or evaluate the information available. In most games there are many micro decisions made about player movement, actions or weapon selection. These micro decisions are important and tend to used to create a skill ramp, they are unlikely to create a good satisfying experience. Long term or strategic decisions can be very satisfying, but tend to be distanced from the 'action' and most players fail to associate the reward with the choice. In the middle are 'tactical' choices, they can be made quickly and in the action with the outcome linked closely to the input. It is these moment by moment 'tactical' decisions that have the greatest lasting impact on the players, and are the choices that the designer needs to focus on most.
For more on decisions in design, see one of the best articles on game design I have read, Greg Costikyan's article 'I have no words & I must design'.
Providing an environment that enables 'tactical' choices minute by minute in a game is crucial and very much overlooked in favor of setting/fiction/narrative/camera and mirco-player controls.
Choice is closely followed by meaningful player rewards (another time)