In the book, Ichabod Crane was a schoolteacher, an ugly, unappealing schoolteacher at that. In this movie, Johnny Depp was neither ugly nor unappealing nor a schoolteacher. Instead, Ichabod was a Goth-looking Sherlock Holmes type detective. He was intelligent and much more appealing than the original character.
Katrina Van Tassel wasn't a witch in the original book or movie. She was cast as a sort of good witch, the counterpoint to her stepmother's evil witch character. The evil stepmother was a bit too Cinderella for me with a bit of Snow White thrown in because the Evil Queen was also a witch. There wasn't a stepmother in the book nor any living Mrs. Van Tassel. As far as I recall, Mr. Van Tassel was a widower who stayed that way.
The book and movie both give the same reason for the Headless Horseman's behavior: the misplacement of his own head. After the Horseman was beheaded during the American Revolutionary War, someone buried him with his head away from his body. He became a sort of pathetic but frightening restless spirit traveling around in search of his head and avenging himself on people who still have theirs.
Very creepy take on an old tale, especially the finding of the head collection under the roots of a very old tree.