Danc at Lost Garden has an interesting post up on themes in games and the effect on game design: while there are definite reasons for the same types of themes and aesthetics popping up in games over and over again, a careful balance needs to be struck between 'skinning' themes and mechanics and putting coherent game play above it all. It's better that a game 'reads' badly from a literary (narrative) standpoint, but makes sense in terms of game play:

In fact, the final theme may be semi-incoherent if you attempt to analyze it as a literary work. However, that doesn't matter because it provides the moment-by-moment scaffolding of feedback that helps the player learn their way through the game. As long as the game is fun and delivers value to the customer we can often toss the literary definition of theme out the window. In fact, you start getting into trouble when you make the theme so rigidly defined that you can't adjust the feedback for specific game mechanics .... The hundreds of little trade offs that occur when theme coherence wins and gameplay loses diminishes the effectiveness of the game. So you can't just 'skin' a set of game mechanics. When you do makes the attempt, a well executed iterative process of game design will often result in a game that is quite different than its source material. A poorly executed process results in a game that plays poorly.

He suggests that designers start working on 'vertical' slices early in the design process, so that they can work on merging themes and mechanics in a way that will make sense over the course of a game. As always, a thoughtful entry from Lost Garden and worth reading. Theme and game design [Lost Garden]