It may just be that "how cool would that be!" is a poor foundation for a game (this would make me a little sad). If this is true, then what is a good foundation for a game? What makes Hell for Leather something people want to play? Why is the Mountain Witch so popular? What is that appeal that keeps bringing people back to Lady Blackbird?
Mostly what I was trying to get at it is that "how cool would that be!" is a general statement that conceals something specific -- there's a reason you think it's cool. In some cases that reason might turn out not to be all that exciting (like sometimes, playing with Lego, you just notice a new way to connect some blocks, and that's really the whole reason right there.) But in other cases, that reason is something more interesting -- something that could be developed into the "reason to play" that you're looking for.
I think most people, especially people who play roleplaying games, come at things from the lego/starcraft, build-it-just-because sort of angle. But there are always reasons they like to build some things, and not others, even if that reason is just 'because it follows out of all the other things I've built and thought about, before.'
I mean, a big part of why people like to play Lady Blackbird is because the setting is so cool (and instantly familiar, genre-wise) -- the illustration of the ship and the sky-squid sparks their imagination, and that's reason enough to play. (It's especially reason enough because the game is presented in such an innovative way, reducing the barriers to play -- the game does a great job of
removing reasons 'not to play'.)
But stuff like The Mountain Witch or Dogs in the Vineyard obviously have other things going on, along with the basic 'cool' of imagining weird things. And probably, in both those cases, the designers had some idea of what that was going to be, even as they started figuring out their games -- but that doesn't mean they had the question completely answered. Probably they refined and modified and even
discovered new parts to their answer as they went through the design process.
So that was mostly all I meant to say -- that it's fine not to worry about the answer right away, and that probably the answer has something to do with why you stuck with that particular 'cool' thing longer than the others. Answering the question for yourself is a good start to answering it for other players.