For me, it's the "atmosphere" of the story. I love to read stories where the characters face almost impossible odds of survival, let alone thriving in their situation. Any crisis will do, but the "atmosphere" of the story must be just right, whether eerie, threatrening, chaotic, etc., depending on what the story is about. For my first novel, I tried to have a theme of uncertainty and the possibility of danger/death "running" in the background. Even when characters believe themselves to be safe, they aren't, and sometimes that truth is hammered home.
So what do you look for in a good book?