When designing scenarios, I tend to perceive two major factors in terms of what I'm making: the imagined world and the imagined session.
So the imagined world is all that simulation-y stuff, by which I don't mean that's it in any way realistic. It's just what we know about the world.
|You know, this kind of thing.|
The second question is one of the imagined session. Roughly speaking, when the players get to Frog Island, what are they going to do there?
|"Get caught by rabbits" is not what you were expecting.|
"Hey, let's go to the Isle of Frogs! I wonder what's there!"
"Could this killing be linked to some deeper conspiracy?"
"Nah, it was the husband."
I have played in games that were so wedded to the setting, that were so happy to be adventuring in Glorantha or the DC Universe or whatever, that we never actually got to do anything. I love Glorantha my own self, but I didn't sign up to carry Kallyr Starbrow's luggage. It makes sense in the world, but even I have my limits.
This is a problem in book design as well. Vampire 2eR was a huge offender in this regard -- page after page after page filled with stuff that was definitely true about the world, but was so vanishingly unlikely to come up in your game that there was no justification for providing it.
Which is not to say that setting design isn't fun or interesting. Good setting design produces interesting session challenges in a way that is quick and easy and feels natural, and the accumulation of even non-game-relevant setting detail lends the events of a game weight that it might not otherwise possess. But I feel like a lot of people do it with more enthusiasm than sense.
Session-focused design has its own problems; I suppose you could say that if you assume you're going to find a balanced, level-appropriate dungeon encounter wherever you go, you might as well go anywhere. (You could say the same thing for a moral dilemma; there's nothing more infuriating than a GM who's going to contrive your ass into a moral dilemma no matter how hard you scheme.)
I could and probably should talk more about the problems of session-focused game creation, but in my experience the thing I see more is design that's excessively setting-focused. I think, and I could be wrong, that this is because a) people enjoy worldbuilding, and b) a lot of game companies (particularly in the 90s?) were publishing games that were intended to be read rather than be played. I speak subject to correction, of course.