Spoilers here:
I loved the book, which always makes watching a film version perilous. But I really thought this version unnecessarily irritating and, although it offered some decent acting and scenery, ultimately painful. It really strips out everything that made the book such a treasure.
The worst parts were the character rewrites. The first serious jar was finding Martha, the sweet and kindly maid, stripped of all her warmth and the generosity that, crucially in the book, first begins to penetrate Mary's selfish shell. Dickson, who in the book is the spiritual guide for Mary and Colin, had none of the glow that transforms them both. And then there's the cheap device of making the housekeeper, Mrs. Medlock, unnecessarily villainous. In the book, it's the entire apparatus of bad medical care, terrified servants and English emotional repression that is suffocating Colin. Stereotypical villains aren't needed and don't appear.
Everything outside the house is off, too. First there's that pointless dog, as though the audience needed one - a crude way to escape the more sophisticated directing chore of conveying the mystique of the robin, perhaps? (The Robin is a CGI thing of unnecessary cuteness.) And of course, there's the garden itself. In the book, it's a walled, overgrown, neglected space that inspires Mary's first discovery of nature when she first digs around crocus shoots to let them 'breathe." Its rebirth in the book - the return of blossoms under their combined care - is bound up in Mary and Colin's finding their own inner lives, all three recovering from sorrow and neglect. In this movie it's a vast, shapeless, rambling space with a pond where the kids, of course, laugh and splash. Forget the mystique of tender growing things; it's basically a child's dream playground. With a big wet dog, how cute.
Even the manor is oddly redefined (maybe to save money on props?). In the book, Misselthwaite is grand and beautiful but closed, dark and oppressive, haunted by sorrow, beauty turned into prison. Here it's a stark, gutted shell, having been occupied by soldiers (why?) and finally goes up in flames - uh, emblemizing rebirth, in case you missed it. Cliches abound in this movie, poaching and replacing all the unique bits and twists that made this book so famous and so loved.
Even the book's famous emotional peak, when Colin runs into his father's arms, is weirdly ducked. The director apparently thought it more realistic to have Colin still struggling with crutches. So much for the emotional pay-off - the beaming lad, the proud father, the astonished servants.
It's always a shame when someone making a famous book into a film doesn't understand or even like the book. The Narnia films were a sin against literature. The Jackson LOTR, after the first film, kept going off Tolkien heights into the juvenile abyss of Jacksonesque story-telling that wrecked his King King. Maybe someday directors will remember what they knew better in the 1940s - that these immortal authors were better writers than they are. If these classics had been written like these movies are written, they would have sunk like a stone.