Some games feature the ability for players to unlock features.

This provides the following benefits:

1. A sense of accomplish between playthroughs.
2. A Smooth learning curve, where additional complexity is unlocked over time.

Not only does feature unlocking provide a smoother learning curve, but it also connects the gates to player achievements. This allows the pacing to be directly tied to player growth. A player cannot experience more complex gameplay until he has demonstrated aptitude with simpler gameplay.