The main problem with the movie is it so very unfaithful to the book. It doesn't even attempt to remain true to what the book is truly about.
First of all, the book is a Christian fantasy, written by a Christian author, Madeleine L'Engle. The movie ignores L'Engle's faith to the point of mockery. To add insult to injury, the movie doesn't even try to add in something that might satisfy in it's place. Not that anything could, but an attempt would have taken away a bit of the sting. Instead, we're left with banal generics about "light," "darkness," and the "universe," with some daytime pop psychology mixed in.
For an example, the movie has Mrs. Which, played by Oprah Winfrey, say "The only thing faster than the speed of light is the speed of darkness." What rubbish! I genuinely hate that line. Theologically, even just morally, it's garbage. Light doesn't have to outrun the dark. Good is not weaker than evil. The smallest speck of light is enough to brighten the most encompassing darkness. That's the message we should be sending children, not some banal quip that makes evil seem stronger than it really is.
The movie's fatal flaw is shown mainly in the characters, but also a bit in how the plot was handled. I'll start with the characters, but I want to say something first. I don't agree with those who say the acting was bad. The diverse cast was mostly wonderful! But, even the most talented actor can only do so much, with a bad script. The cast did the best that probably could be done with what they were given. Sadly, what they were given was trash. That isn't their fault though.
Chris Pine played Alex Murray, a character horrible misinterpreted by the movie. In the book, Murray is a renowned and VERY well-respected scientist, not some outsider on the fringes of his field. This is a man who is consulted by the President, at times! His work with the tesseract was a group effort with other scientists, not just his wife.
The movie utterly trashed his motives for developing the tesseract. He didn't want to "shake hands with the universe," whatever that means. Murray in the book was a man of faith, working for the good of his fellow men. The tesseract was supposed to help people, not just fulfill some personal agenda. Murray in the book took risks, yes, but as part of his responsibilities to his team and country.
Equally flawed was the movie's version of Mrs. Whatsit. In the book, Mrs. Whatsit is, frankly, an angel, who used to be a star, before sacrificing herself in a battle against evil...a battle she won. She was a fun character, though, playing a game at being human. She dressed as an old tramp and stole sheets. The sheets in the book, though, weren't stolen to create some magnificent robe for her to look beautiful in, for her vanity. Nope. In the book, she took the sheets to make cheesy ghosts, in order to scare people away from the haunted house.
The worst thing about the movie Mrs. Whatsit was how she treated Meg. Consistently and frequently through the movie, Mrs. Whatsit bullied and belittled Meg, showing intense dislike and openly wishing she wasn't there. What an incredibly terrible message to send! This is supposed to be a wise leader, teacher, and guide, but that's how she treats a child? Compare that to the book, where Mrs. Whatsit is playful and gentle, where Mrs. Whatsit's assurance of her love is instrumental in Meg defeating It. I'm a fan of Reese Witherspoon, but she was so wrong here. Frankly, Kathy Bates would have been a much better choice.
Another character who should have been happy, playful, and kind was the Happy Medium. Zach Galifianakis played the role as a grump, for whatever reason. Frankly, this is the role Michael Peña should have played. His comedy style would have been perfect for a well-meaning seer trying to show the kids comforting things, their mothers, and instead showing them scenes of pain, as it was in the book. I don't know why they got rid of this bit of character development in favor of CGI crap about balancing on rocks. It's just one time of many, where something deep was replaced with something shallow.
Zach Galifianakis could play the Man with Red Eyes, if he can pull off legal evil, a creepy bureacrat, with a deep sense of menace, highlighted by a fake cheerfulness.
Finally, we get to Charles Wallace. And, yes, he should be called that, not Charles or Charlie, no. He's Charles Wallace, because it shows he's not ordinary. In the book, Charles Wallace is a gentle, thoughtful child, who very often knows what his sister is thinking and feeling. He's a boy who listens. This is a very important part of his character. He has a wisdom to him that is far beyond his years. He's not a typical kid who screams and yells, when someone is being mean to his sister. Charles Wallace barely talks in front of other people at all! There should be something otherworldly about the boy. Instead, McCabe was directed to play him as a bit wild and mouthy and Charles Wallace's wisdom, understanding, and intelligence were barely shown at all. Not only is this a sad waste of his character, but makes the sequels impossible to get right. They made the classic mistake of telling us Charles Wallace is special, instead of showing us.
Poor Calvin. They didn't so much get his character wrong, as fail to give him character at all. He was just undeveloped. Of course, they ruined the possibility of a good adaptation of A Swiftly Tilting Planet, the third in L'Engle's trilogy, by changing his home life from a poor family, with lots of kids, to him being an only child with just a dad. The only character treated worse was Aunt Beast, who was left out entirely, which is shame. Well, they also left out Sandy and Denys, the Murray twins. A single scene with them would have been just fine.
As for the plot, I think a single example will highlight how wrong the movie went. In the book, the children arrive on Camazotz. They walk through a suburban neighborhood and see children "playing." The boys are bouncing balls and girls and skipping rope, all simultaneously to a certain "rhythm." Yes, the movie almost gets that much right. The children see that one child alone is out of rhythm, dropping his ball and unable to keep in sync. When the mother's call the children in, the errant child leaves his ball behind. They try to return the ball and the mother is terrified. She assures Meg and the others that there haven't been any "incidents," in a long time. The reader sees that although everything looks neat and tidy, no one is really happy. Submission to evil brings terror and pain, not joy.
This deep, potentially moving scene is reduced to make room for a shallow, unnecessary scene where a CGI forest attacks the kids.
Speaking of out of character, the entire planet of Camazotz is exactly that. There would not a beach scene like that on Camazotz. Camazotz is legal evil, all the way. The evil of Camazotz does not play those sorts of mind games. It doesn't so much try and confuse you, as try to get you to confuse yourself.
In the movie, the children are given a meal, by the Man with Red Eyes. They are given some sort of nutrient base, but the Man can create the illusion of a turkey dinner. Charles Wallace, however, can't be fooled, and knows the meal is shapeless and tasteless, like sand. They weren't actually given sand to eat. The meal did what a meal should do, at least on the surface. It provided nutrition, but without real joy. It was body fuel, with everything better about it drained away. For me, that's the perfect metaphor for this movie, as opposed to the book. The movie tells the story and kills some time, but with all the real joy of the story neglected and ignored.
I really wished they had done a good job. I wanted to like this movie, so much. When you make a beloved novel into a movie, you have an automatic audience in the fans of the book. Integrity demands you give that audience what you are promising them. This movie fails to deliver.