This is more of a personal belief than a universal idea, but I like my concepts to really pop. The best concepts actually spark some else's imagination: they make the average person say "Yeah, I can see that." The hallmark of a good concept is when a layperson hears it and begins pitching ideas off of it. For instance:
ME: "He goes back to 1969..."
OTHER GUY: "Oh wow! He could watch Superbowl III!"
In my life I've sold two projects:
1. Taliban vs. Marines vs. Zombies!
2. Kung Fu High School!
Right now I'm working on a project called HOT ALIEN CHICK! Hopefully this will be the project that CHANGES MY LIFE FOREVER!