Can these be discussed here (sans systems) or is this primarily a mechanics forum?

If you are committed to designing a game, and know what you want the game to do then Setting strikes me as just as good a place to start as System.

Unless you are designing a system (eg gurps) then you will have a setting somewhere, and i can't see where else you would start really. However that wasn't the point of the question.

Okay. What was the point of the question?

Hello,

I shall clarify. This forum is specifically for discussing any aspect of a role-playing game which is in fact under development for publication.

"Publication" is very broad, and includes free posting on websites, or nearly anything else that spreads the game past your own circle of friends.

"Under development for" means that you don't have to be anywhere close to publication - just that you would like to.

To be absolutely clear: starting with setting is just fine. However, do be aware that it's one of the most difficult angles to approach design from, and that many folks here will be interested in system stuff too, and will probably ask about it along the way.

Best,
Ron