Dobby and Kreacher.I was really angry when they didn't show the scene that happens at the end of the book with Kreacher proudly leading all the House Elves in the Hogwarts kitchens to help in the battle.
Kreacher's character development should have been in the movie. At the very least they could have given him Regulus's locket! Just simple kindness and affection can go a long way. Look at how Kreacher behaved when he was shown love.