Obviously, since I am claiming there is a specific interpretation of the question, that it is not as straight forward and easy as it may first appear. If you were to ask me this about, say, Legend of the Five Rings. The answer wouldn't be "It is about samurai in a fictional setting made by combining elements of Japanese, Chinese, and other asian mythologies into one." By the same token, Houses of the Blooded is not about "Nobles in a fantasy land, playing their games of power, intrigue, deception, and revenge". Sure, technically, that is what those games are about, but it isn't what they're really about. (Oh, if you're wondering, I am using John Wick's games because I know he uses this question, and it isn't hard to see the answer when you look and know).
So, what do I mean? Or, to be fair, what does Mr. Wick mean? He means one word. In one word, what is your game about? What is its core theme? The thing that the game, and its mechanics, is designed to give and show? What is the core element that should be used for stories told with that game? In other words, and as said, what is the game about? L5R in this light is about honor. Houses of the Blooded is about tragedy. So, what is your game about?
Now, what do you do once you know what the game is about? You need to build off of it. There are more questions to go with it, and I'll go over them one by one, but the core of the answer is you make that theme part of a core mechanic. The honor stat in L5R represents this, using your honor as the carrot and stick to guide people into playing out the game and the stories (sadly, with 3rd and 4th ed much of this has been lost and diluted, so to really see it you need to go and check out 1st and 2nd ed.) How you gain Style points in Houses of the Blooded is how Tragedy comes into play, you get to be cooler and more bad ass, the more you fail tragically, the more you hurt.
So, if you're making a game, or thinking about making one, take some time and think about what the game is truly about. What you want to be resting at the heart of the game system, and the stories told with it. And, hey, if you don't mind. What is your favorite game system, or your work in progress about?
For me, well, I already told you about Legend of the Five Rings and being about honor. Near as I can figure, Dark Heresy and the games in that line are about inevitability. Inevitable failure primarily, with mechanics set up so that you will fail at some point, it is just a question of when, how long you can push it off for, and if you'll survive it when it happens - or at least die well. Now, my work in progress is about sacrifice. This is where my mistake was made, as if you look back to the game design journals for m.a/c.c (nope, no links, sorry) I thought it was about commitment. Which, it is in a way, but the game's main theme is sacrifice.
How about you?