These animated GIFs from Disney's upcoming film "Frozen" show the comparison of shots from the old trailer next to the shots from the new trailer. Obviously, they did a lot of reworking and "plussing" to these shots to take them to the next level. It is a great example of how taking the time to go the extra mile can make a huge difference. Even the most talented artists and animator in the world will still go back and make changes to their work. I believe these artists are the best in the world for that reason: they put in the extra work to push themselves when most people would have called their first final draft good.
For me personally, changing a shot after I've worked so much on it is one of the hardest parts of animating. It's crazy to think that even extremely talented animators at Disney still redo work and throw away work all the time.