Day 3 - What gives a game "staying power"?
An interesting question, and one where I don't really know what "staying power" means in this case, but...
If I had to take a stab at it, I would say that what gives a game staying power is something that is intangible, known only to the audience at large. It could be a game world that resonates with players and GMs alike, it could be an overall "experience" of/with the game that makes it stand the test of time, heck it could be just that sensa wonda that players and GMs alike experience with the game.
If nothing else, I look forward to seeing what other folks come up with in answer to this question. :)