Genre
Genre is primarily about the game content: what type of game is it? Is it RPG, first-person shooter, adventure, or any other type? Genres can be even narrower than this, such as fantasy MMORPG and cyberpunk adventure game, or competitive deathmatch, first-person shooter. Sometimes, you'll want the genre to be very specific, and other times you'll not, depending on your aims. Be specific when building a game in the truest and most genuine spirit of a traditional, well-established genre. In this case, the idea is to do a good job at a tried and tested formula. In contrast, avoid too narrow a definition when seeking to innovate and push boundaries. Feel free to combine the existing genres in new ways or, if you really want a challenge, to invent a completely new genre.
Innovation can be fun and interesting, but it's also risky. It's easy to think that your latest idea is clever and compelling, but always try it out on other people to assess their reactions and learn to take constructive criticism from an early stage. Ask them to play what you've made or to play a prototype based on the design. However, avoid relying too heavily on document-based designs when assessing fun and playability, as the experience of playing is radically different from reading and the thoughts it generates.
For Dead Keys, the genre will be a cinematic first-person zombie-typer! Here, our genre takes the existing and well-established first-person shooter tradition, but (in an effort to innovate) replaces the defining element of shooting with typing.