Usually, I think of endings as tidy and relatively lowkey: all the drama from the book is tied into a sparkly little bow, my shattered feelings are glued together until next time, and everyone live happily ever after. Except for the villain, of course.
Sometimes this works. Sometimes it’s overdone. As human beings, how often does a chapter of our lives close that tidily? Even after something as major as the death of a relative, there’s still a lack of closure. The funeral. Making arrangements. Maybe an awkward family reunion of sorts, when everyone gathers for the funeral. And then emotional closure may not come for weeks or months.
Wrap up the main plotlines.
If your story is a standalone romance, not letting us know who ends up with who is cruel. It may induce passionate romance readers to throw your book against the wall. To avoid instances of book cruelty, please avoid this. If your story is a dystopian rebellion, make sure we know that the arrow-slinging heroine is the one that wins in the end. Marking out which plot lines are major and which are minor is really helpful.
Leave a sense of uncertainty.
Mimic real life. If your MC just overthrew the Big Bad Government and took over as leader, there’s bound to be uncertainty, chaos, hope, fear. Instead of everyone in the country joining hands and singing, leaving off on a note of uncertain hope would be appropriate.
Avoiding overkill happy endings is also important. Respect the traumatic sausage mill that you have stuffed your characters through. Nothing will ever be the same. If things go back to normal (or an even better “normal”, such as a strong friendship or a cute romance), then it doesn’t feel right. You can’t brush over past trauma with a big eraser. Maybe your ally deals with PTSD. Maybe your character has to learn to fit into her labor-intensive community with one arm or leg.
Give them a life before and after.
Backstory is harped on a lot, usually because it can make a big difference between flat and 3-D characters. Sometimes writers forget to make sure that it feels like they still exist after the book. They do have lives that aren’t being chronicled in story form, and making it seem otherwise can distance your book from reality.