A professionally-written story will not necessarily sell. The prose can be good, the characters interesting, the plot possibilities mysterious, and the ending can kill it all if it’s cliché, or goofy, or low stakes. From TV, consider the infamous Dallas fiasco when a whole “season” turned out to be Pamela Ewing’s “dream.” Consider the miniseries It where the monster turns out to be a “giant spider.” Consider the story where the protagonist realizes at the end that he's been “dead” all along, or the tale where an abusive husband seeking to cheat on his wife meets a woman at a bar who turns out to be a…wait for it…vampire.
There’s no “payoff” in these endings. The “it was all a dream” is actually an insult to those who have invested their emotions in a work. The “guy who discovers he’s dead,” or who cheats on his wife only to meet a “vampire,” is too easy. The “giant spider” is just lame. Modern readers especially want more.
I mentioned one of my unsold stories the other day, “A Curse the Dead Must Bear.” I think it hasn’t sold because the ending doesn’t pay off. A man falls down the stairs and is declared dead. He seems dead, but his consciousness continues. He is aware of everything around him but cannot escape the decaying prison of his body, and he finds that every other “dead” person in the cemetery with him is the same. He can hear them murmuring while, of course, the “living” hear nothing. He begins to hate the “living” and is comforted by only one thought, that they will soon be in the same situation.
Big deal! I thought the idea that every dead person’s consciousness would be trapped within their decaying shell was interesting, but that point is revealed midway through the story and other than that what is the reader's payoff? The protagonist cannot “do” anything about his situation. He can only hate, and the target of his hate is perfectly safe. This is “low stakes.”
The cheating guy who meets the vampire is low stakes, as well. We can see that ending (or something like it) coming a mile away. And, two, the guy is getting what he deserves. Other than a little “he had it coming” feeling the reader isn’t getting much emotional payoff. The ending doesn’t leave us particularly happy, sad, disgusted, or afraid. It leaves us flat.
Want another example? A guy plans to commit suicide because he can’t stand his life. He wants oblivion. But when he finally gets up the guts to do it he finds that his consciousness continues and he’s in Hell. End of story. And low stakes.
How do you avoid low stakes endings? One way is to consider the low stakes “ending” as the beginning instead. Cheating guy meets vampire at the “start” of the story, and gets turned. Goes home to torment the wifey, thinking about how much fun he’s going to have now that he’s a supernatural abuser. He finds that his wife is having an affair too. And her paramour is a werewolf who, shall we say, “rips abusive husband a new one.” Or maybe he finds his wife having an affair with the female vampire who turned him, and they only want him to watch. Or maybe… Well, you get the idea.
You have to ask, is the ending to your story worth the effort that the reader has put into it? Will they go "cool," will they shake their head in disgust, or will they just not care? You only want the first result.