This keeps getting more and more complex.
At this point in the class, I know a game has to have some sort of goal and objective - some people argue it can be user or creator defined.
I feel like it has to include at least some definable set of rules, or at least a set of regulations one must follow in order to achieve that above mentioned goal.
I also personally think that a game has to incite enough interest to keep the player or user motivated to keep on playing. Of course, this comes down to personal opinion and interest, which is why there are so many bad games.
But taste is beside the point.