I've been reading Kendare Blake's second Anna book, Girl of Nightmares. I loved the first book Anna Dressed in Blood to distraction. I'm a sucker for a great horror story, and Blake has proven that she's got the chops to write, not just a scary book, but a whip smart scary book with a sense of humor and a lot of heart.
Though only 1/5th of the way through the second book, it's proving to be equal to if not better than the first. But what I've noticed is the engine that drives the book. It's a book about a guy who kills ghosts, so you'd think that ghost killing would be that engine, but it's not. That's not what's driving these characters along. It's something far more human.
In Deathday, I originally had the letter as the engine. The looming specter of death lashing poor Ollie through his last 24 hours. But my agent at the time and my editor both helped me see that the real engine driving the story was the relationships between all the main characters. The letter allowed me to set some wheels in motion and helped grease them along the way, but the real engine that drove the story was Ollie and Shane, and Ollie and Ronnie. The need to settle these relationships was what kept Ollie moving from one event to another.
Similarly, while Cas IS hunting ghosts, it's his connection to the titular Anna that drives his story.
Even in a big, high concept book like The Hunger Games, the games are merely a backdrop. It's Kat's need to return to her sister that informs every decision she makes.
So I guess what I'm saying is that a high concept isn't enough. There's got to be more. Conflict and characters we care about. I suppose that's the difference between a high octane 8 cylinder and a moped.