Outdated, go here.
Recently, my posts have said "the rule set is the only thing that matters".
It's really a misleading statement, of course. It's kind of like saying "phsyics is the only thing that matters". Sure, everything relies on physics, but things like food and air and Wii are the things that everyone cares about. The fact that physics makes these possible is a pretty remote connection to most people.
Games are the same way. The pretty graphics, the hopefully-interesting storyline, the cool weapons, the level design, the actual play of the game... it's what players live and breathe. But all of that stuff either arises from or is brought to light by the rule set of your game.
You have a villain named Murmur. She's a kickass design with some great lines and some fun superpowers. Plus, she looks really hot in what little she's wearing.
Of course, if you've designed her without thinking about the game rules, much of your design is wasted. For example, if the game is a real-time-strategy, she's going to be about thirty pixels tall and her delicate outfit won't have any more punch than a generic bikini. On the other hand, hair that is bright pink is fine in such a scale because it sets her apart, but if she were portrayed larger, it would just be loud and ugly. (Why the heck does a villain named "Murmur" have pink hair?)
Her fun superpowers must be adapted depending on whether it's an RPG or an FPS, and that's going to be an ugly process. Even her lines will be portrayed differently depending on the game - a game with no facial animations or really tiny faces (like, say, Deus Ex) will leave every line flat, totally changing the way you have to write. Is it recorded over audio or simple text? That also changes the way you write.
The parts of Murmur and all the other things that impact the player rely on the rule set. The overall rules of the entire game, not just the tiny piece you associate with gameplay. With a different rule set, the same character has a dramatically different feel. The same event is a totally different experience. Even the same art gives an entirely different impression.
Content is king, sure. Without content, nobody cares to play. But what content gets crowned is up to the rule set.