A Sense of Progression: Keeping your game interesting

Keeping a strong sense of progression really adds to gameplay. If you're not careful, your game could become just one boring level after another. A sense of progression is a way of rewarding the player as he or she progresses along the game. You might think that you need to overhaul your game or add tons of code to have a nice set of rewards, but so isn't the case! I will present some simple, easy-to-implement rewards that add much depth to a game:

Different tile sets:
Levels that look and feel different from one another will keep them interesting. Even simple things such as changing the colors around or making one tile-set more worn-looking than another add to the feel of the level.

Spreading out enemies:
If you have a handful of enemies in your game, don't throw them all into the first level. Save some of them so they can be introduced in later levels. So, for example, level 2 won't just be known as just "level 2," but rather the level that introduces that scary monster!

Spreading out weapons:
Same as above, but with weapons.

Secret areas:
It's a nice feeling when you stumble upon a secret area in a level. Hide some in your levels and stuff them with goodies to reward your player when he or she finds it.

Story:
I'm not talking about adding complicated dialog or cut-scenes. Think about this example for instance: level 1 is a forest, level 2 is a castle, level 3 is a dungeon, level 4 is a lava cave where you fight the evil demon. That scenario has a type of storyline progression to it; each level leads to the next. It indirectly tells of a hero venturing through the forest to find the castle that leads to the evil demon that he or she must annihilate. Try to think of a scenario more original or creative.

Conclusion:
So you see, you can add much depth to your game already using what you already have. These are just some of the basic ways implemented in most games. Try to think up other ideas than what I presented here to make your levels more interesting and to add a sense of progression.