Having a clear zeitgeist for the game seems to be a very important design component and whatever mechanics exist need to support that. One of the reasons I often get frustrated with folks who say your "RPG (usually D&D) can be anything you want. It can be about love and hate and family and..." is because it cannot be those things, for those things are not supported by the mechanics. The mechanics have to at least facilitate the kind of play and support the tone of play that the game explicitly talks about or dissonance creeps into play. And certainly at the end of the day if folks are having fun, maybe dissonance in play does not matter to them. I just think it should matter and as a designer it matters to me.