This is a kind of question that a few people on the main site think is off topic for this question, but wasn't actually discussed in the definition phase (at least, I can find no such question there).
Is it really off-topic here?
I say that yes, it should be off-topic.
I mean, a question like "why does Team Fortress Classic has separate health and armor values" is closer to a valid question, in my opinion - an example answer is "to introduce a limited friendly-fire mechanism", or "to give more importance to the engineer".
But I feel that a general questions like "why do FPSs sometimes have separate health and armor values" is a lot more about general game design. There's a separate Area51 proposal for game design and development.
Also see another question here on gaming meta, where my FAQ proposal explicitly prohibits game design questions - although it's just a proposal, of course, it's not anything official.
There needs to be a fine understanding about what is and what isn't a Game Design question. A game design question is one that is concerned about the development, implementation, and motivation behind a mechanic. As our site is geared towards gamers, whose main concern is how this mechanic affects the experience and not its creation, it is not really in our field of expertise. We can only guess what the developers were thinking of (those of us who aren't also game developers, at any rate).
A question about the mechanics of a game, in terms of the game world, is not a game design question. It's asking about what is happening in the game, as opposed to what is happening in the design of the game. For example, "How do effort values work in Pokemon" is not a game design question because its interest is in the gameplay aspect. Comparatively, "Why did they change from the type-attribute to the physical/special split" is a design question, because its interest is in the choices made in creating the game.
So as a good rule of thumb, think to yourself when it comes to a mechanics question, "Does this information concern me for my choices in playing the game?" versus "Does this information concern me in how the game was created?". The latter is off-topic, the former is on-topic.
In the most generic sense, I believe it is off-topic. General Game Design questions seem like they would be better suited for the Game Development SE, that will be entering private beta soonish.
If its about the underlying design of an existing game, I believe that it is well within the scope.