I voted setting. For some reason the first thing i thought of when you mentioned this thread was "things you can learn from playing or running setting X and use in setting Y."
For instance, what can you learn from a Star Trek game where the PCs are the senior officers - including the captain? Doing that sort of game well requires (among other things, I'm sure) roleplaying the superior/inferior officer relationships without douchebaggery, accepting the power differential between the ranks in a way that's fun to play rather than annoying, utilizing the specialness of the senior officers' departments and expertise...I could probably go on. How could you apply those lessons, those necessities, to another genre...or even the trappings and assumptions and coolness of one setting or genre to another.
Even as I wrote that i caught myself phrasing it in a more generic way than I intended, so that it sounds less Trek-y and more "how to have a well-balanced group".
Anyway, that's what popped into my head. 'Cuz my head is like that.