What moved me the most was just how "human" the old Vader character was portrayed. The mysterious evil lord in the wicked black suit started as a young man afraid of losing the love of his life, who was willing to do anything to save her. Even if it meant going against all the teachings and morality of the Jedi way.
Look at any extremist group (including some religions) and see the kind of people that they attract: those who are afraid and those who are weak or helpless. The appeal? They offer an answer, a solution, company, understanding, and strength.
This whole idea and the "tragic" fall of Skywalker are the most powerful aspects of the series. I love the symbolism Lucas employs and the much broader message he delivers. Anyone who thinks Star Wars is about a young, eager filmmaker's passion for building futuristic models and blowing stuff up has completely missed the boat.