Beyond the new details of the plot there is some thing interesting about what JK Rowling has done with the invention of the villain Lord Voldemort; she has created someone so terrifying that characters refer to him as "he who must not be named." Voldemort is such a complex villain with a dark and twisted past. The books do a much better job of painting a much richer deeper picture of Voldemort's past. To be fair the books never intend to make the reader think there is chance he will be good, but they do make you feel sorry for him. In the fifth film Harry says to him "You will never know love and I feel sorry for you."
Voldemort's return to his full capacity signals a war that is brewing. Both the films and the books do a good job of pointing out that there is nothing that is definitively good or definitely evil. There are shades of grey, and how people choose to act defines which guides their actions. What if the sorting hat put Harry in Slytherin? Would the story have changed or would Harry been the wizard he was always meant to be? Do labels matter, or do actions define people? Rowling is great at creating this battle between good and bad the light and the dark, but she also states that labels don't matter! The wizards who come from pureblood families like the Malfoy's or black's were mostly dark, then there are families like the Weasley's who use magic for good. There are so many complex issues about good and evil that come out of these books.
In the end though beyond the use of the imperius curse the wizards and witches that are bad, are bad!!! Voldemort is a true villain; he split his soul my killing other people in order to live longer, and murder both muggles, and what he would refer to as "mud bloods." At the end of this film his return brings about the fear that there are few who are safe from his wrath, especially our main hero, Harry Potter.