[...] a core gameplay idea, which is well defined, but also flexible, along with completed art assets; they don't have to be "production quality", but they should be good enough to make people think: "... Ok, I believe that he could polish this into something market worthy.".
You could also try making screenshots: You don't have the game, but you can paint a screenshot of what you have in mind for the final product. Or, even better: Make a 10 second animation that clearly shows the basics of gameplay.
Best part of a post I've read so far, a personal opinion of a physically achievable goal to set as priority for most any designer when posting.
Wait! You aren't job? I'll try to more thoroughly read before I post next time. I appreciate it. Be good at you are job, heh. Common the latest sit down between John and Ben Burtt would be pretty rad and it would have me on board for an indie game faster then most any game design idea, but maybe I'm shallow that way. Think of Wall-e! God that movie sounded good.
@Cornstalks That's the kind of folk I like to work with too.
I'm hoping to hear personal accounts of classified posts that stood out, indie devs that got your attention not just because of the ideas they presented but because their post painted a whole game worth making (in your opinion). I want to know the elements in those posts that made the difference. Maturity, professionalism, explore the box sort of aspects are pretty drilled around this forum (for good reason) but occasionally there must be designers that make you excited about games and I'd love to hear about it. The suggestions made by Goran are very applicable to my situation but there must be more out there. Maybe game or genre specific examples of "raising the classified posting bar" or plain old creativity in a classified post worthy of mention.