When gamers talk about a game which redefines a genre of games, "Thief" is the type of game they mean. It's not just playing the same sort of game in a new way, it's taking the core concept of a genre, and going in a completely different direction. It makes a new game out of a familiar concept. In one elegant stroke, "Thief" takes the concept of the first-person shooter and completely flips it around, greatly expanding the potential of what an FPS game can be.