Game design is the process of designing the rules and content of a game.
The depth of a game can be increased by adding new entities (items or enemies for example). However these entities will only enrich the game when they differ from existing content, so they need to not only look/sound differently but also 'play' differently. Sik0fewl explains:
"Any enemy, action or item added to the game should serve a specific purpose, thus allow something not doable by other items, enemies or actions. Different looking actions that serve the same purpose are most of the time just confusing and add unnecessary bloat to the game. There is of course a little need to have a variety of enemies, but even with them variety in behaviour is much more important then variety in look. Different looking enemies might make pretty screenshots, but different behaviour makes a better game."
- No useless items/actions/enemies from Rejected SuperTux ideas