I feel I've gotten quite good at knowing how to pitch campaigns. And because I feel like it took some time, I also feel it isn't easy, and just 'be explicit' may not be helpful.
There seems to be a divide between the old I have a setting and I want to use D&D to run it, which is my preferred approach, and let's play D&D, the setting is whatever.
I've found that if I just say what I don't want, I'll still get something I don't want, it will just be something it hadn't occurred to me would not fit. More to the point if players are pitching something that doesn't fit and you're turning them down there is a failure of communication. Clearly the players are failing to get what you want and there's a good chance if the player who has the clearly innappropriate character doesn't get it, the players who seem to have a character that fits doesn't get it either
Now it may be because your concept is too narrow (and I do find that campaign ideas tend to start narrow but then if I give them time I can begin to see how to widen them). But really it doesn't matter if the concept is narrow if the players get it and want to work with it.
So a lot is about communicating tone and also communicating setting (without doing the unbelievably dull history dump of 1000 years ago the Agathian empire invaded the plains of Seth" seriously who cares?).
A lot can be done with
13th Age icons. They're a genius idea for communicating setting and tone. Who are the major players? Where are they? What are the connections between them. A paragraph or two each, an image stolen from the internet and you communicate an awful lot of what you're aiming for, if you have the "High Inquisitor" as an icon who's a religious fanatic who burns witches and heretics, then it seems you're going for something of a grim warhammer like tone. If you're asking them to define how their pc feels about these things then you're immersing them in the setting and getting them to consider their character right away as something in the setting. (You don't have to use 13th Ages relationship dice rolls, just getting them to define their relationships is sufficient). You get characters that fit the setting. "My Ranger is a former witchhunter, he has a negative relationship with the High Inquisitor because he refused to burn some children he knew were innocent of trafficking with dark powers and instead led them to safety."
To use two terms from education, I think there are two issues that lead to miscommunication. The first is the "Curse of Knowledge" you know what you want and it is clear to you - in fact so clear that you have a hard time imagining what's it's like to not understand what you're aiming for or what your referent points are. The other is lack of "Schema" a kind of mental model that helps the players understand the kind of game you want to run and how the pieces fit together. So what you need to communicate needs to overcome the former and build the scaffold for the latter.
