Question: 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?

Answer: 
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](http://area51.stackexchange.com/proposals/2825/game-development).

Also see [another question here on gaming meta](https://gaming.meta.stackexchange.com/questions/226/what-should-our-faq-contain), where my FAQ proposal explicitly prohibits game design questions - although it's just a proposal, of course, it's not anything official.
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. 

For example: [Which is better for Pikachu in Pokémon Red: Swift or Slash?](https://gaming.stackexchange.com/questions/1452/which-is-better-in-pokemon-red-swift-or-slash)