The not-so-good: The PG rating is for "some scary moments and mild language" which leaves the area of discernment wide open for parents. Obviously the rating is key in giving this movie the "appearance" of being "family-friendly" and thereby guaranteeing parents will take kids of all ages, delivering the huge box-office expected for this movie. Parents need to be aware that this movie isn't a "light-hearted" children's story with a traditional good vs. evil fairy tale theme, but is in fact (by Rowling's own admission) a dark battle of good witchcraft (by definition a debatable term) vs. bad witchcraft, or good vs. evil within the world of witchcraft. That is the point parents need to weigh in trying to decide if this movie is appropriate for their child. The story is obviously about magic, which is in every scene, and a few scenes will be intense even for adults, never mind a wide-eyed 6-year-old who can't differentiate between what's real and what's "movie-magic."
My assumption is that much of the audience has already read the books, so I don't believe I'll be giving too much away by warning parents about the different magical levels. If you want to be completely surprised, read no further.
There are numerous imaginative scenes (too many to mention) for all ages: a talking hat that selects where the students should live, a cat "shape-changes" into a woman, friendly (and funny) ghosts fly around the school and talk to the students, a game of ball is played on flying broomsticks, a mirror reflects what you want most in life, staircases switch in mid-air, dining tables are loaded with wonderful candy and scrumptious treats, owls who (no pun intended) deliver the mail, and my personal favorite, a cloak that makes you invisible.
But there are scenes that go beyond "kid-friendly" in their intensity: a bank is filled with evil-looking goblin tellers, a giant grotesque troll corners a girl in a bathroom and tries to kill her, a giant three-headed dog guards a secret in a forbidden room, black snake-looking vines try to choke Harry and his friends, a dark forest contains a friendly but ugly half-man, a half-horse creature warns Harry, and a giant chess game uses pieces that kill the players.
Some dark scenes are necessary to convey the evil force Harry has to battle: a shadowy figure in a huge dark cloak drinks the blood of a dead unicorn and exposes its sharp fangs to Harry, the back of a man's head contains an evil creature, and Harry touches a man who turns to stone and explodes.
There is a long-standing debate among Christians about seeing this movie, or even reading the books. Many would argue that the influence of witchcraft wrapped in such an appealing package (a magical school that teaches spells and incantations to children who perceive these as fun and powerful) might, in fact, entice children, teenagers or perhaps even adults to explore or embrace elements of that religion. The fact that the characters are removed from parents (the only parents shown are silly, bullying or cruel) and told that their classmates and professors are their new "family" only adds to the subliminal message.