Dialogue and audience Well, most comments here have already mentioned the great things about this movie (its music, acting, accuracy), so I just want to put in a word for something I *haven't* seen mentioned yet: the dialogue of the film. Nearly all the dialogue is directly taken from letters/speeches by the actual Founding Fathers, and whatever isn't direct quotes (such as the songs) is paraphrased. This is especially true for John Adams' dream sequences with his wife Abigail (one of the greatest love stories in history) The writers wanted to present Adams, Jefferson, Franklin, and the others as truthfully as possible -- and they succeeded!
A side comment: the film isn't just made for Americans. I recommended it to some of my friends in India, and they loved it. You don't even need to know anything about American history or the Declaration of Independence to "get" the movie -- it stands on its own!
Another amazing thing about this movie is its drama. You know from the beginning what's going to happen, but you're still caught up in the drama of the film.