I had an interesting discussion with one of the other Electric Spec editors lately. We both had an opportunity to read a new short story. I was charmed by the characters and steampunk world. The other editor did not enjoy the story. He essentially said, 'There's a loose thread, an unresolved story point.' I realized he was right. The protagonist released something into the world that had the potential to destroy said world, and didn't get it back. I'd been so distracted by other story elements I hadn't even noticed! Usually, I'm a better reader than that.
So, my advice to authors: avoid loose threads. Don't inadvertently, accidentally, destroy your world. Of course, if it's intentional, that's another matter entirely. :)
Good luck with your writing!