I think what makes a game is the interactive feedback loop, where your interaction changes the state of the system, it's processed into a new configuration which requires further feedback, and there is some kind of objective between agents (player, machine, state of a stack of cards) in a conflict with some kind of quantifiable end point. Simulations and RPG's are more confusing to pigeonhole as games in this definition, unless you consider particular goals as a quantifiable end point (have your sim city reach stage X, or to beat a certain mission in an RPG, or to get your level beyond Y. (Thanks, Rules of Play.)