I was a little in shock, and couldn't remember the movie right way, except that I liked it. A few minutes later, the reason it didn't win came to me: a completely stupid, improbable, sub-plot. In the middle of a wonderful, well-written, well-acted, well-directed film, there was a subplot from hell that belonged on the cutting room floor. Unfortunately, the subplot was so bad, it dragged the rest of the film down with it.
So whether you're facing picture lock, or the final draft of your script, keep an eye out for that tacked on storyline that doesn't belong. Trust your characters. Trust the emotional engine of the story. If you feel the need to introduce some kind of outside influence on your characters, try cutting it and see if the story still stands up. If it does, then keep the cuts on the floor.
Your film will fly higher without the extra weight.