There are three things a good story needs, A beginning, a middle, and an end. Every single book or movie I have liked had these things.
What got me thinking about this was I watched Spiderman II the other day. I had seen it before but for the life of me could not remember what happened. After watching it again I still don't. But I did notice the profound lack of a beginning or an end.
I have liked books that switched things up a bit, started at the end and had the beginning in the middle and that sort of thing. But they still had a beginning and an end. When a story doesn't have all three elements it is not a story. It is just looking at a characters daily life, even the most interesting of characters can't stand up to that sort of peeping.
I've talked about this before, this is the event that changes the character's life and gets the ball rolling. Fred going to work at an accounting job is not a beginning unless he finds something strange that changes his life that day.
It is the same with Superheroes or Monsters. Batman patrolling the streets of Gotham City, isn't a story it's part of the neighborhood watch. His running into a new supervillian that makes him have to change tactics is the beginning of a story.
Stuff happens to move the main character towards the end. Without an end there is no middle.
Not every single plot thread needs to closed, and evil doesn't have to be punished but there needs to be an end. Even in the middle of a trilogy there is an end to the storyline started that second chapter. EMPIRE STRIKES BACK is a great example:
Beginning: Our heroes are going their separate ways.
Middle: They learn they need each other. It's a handy lesson for Luke.
End: They vow to reunite.
No matter what genre you write in, except free form poetry, make sure your writing has a beginning, a middle, and an End.