The biggest difference I have spotted so far between film and book is the character of Inman.
In the book he is ISTP: that is the personality type of a craftsman or gunslinger. Emotionally reserved, physically competent and individualistic. Clint Eastwood or Soolin (in Blakes 7). In the chapter I just read he compares fighting to woodworking, a craft skill. This is almost textbook ISTP.
But the film Inman, as played by Jude Law, is ISFP, the introverted sensual: like Marilyn Monroe or Morrissey. He's vulnerable, passive and modest. I'm still half way through the book, so the two intepretations may converge.
For example in the book Inman meets Ada as follows: all the young men are boasting about how they are going to ask her out. Inman doesn't join in, but goes to see an old woman who knows Ada, and offers to plough a field for her if she will introduce him to Ada. This shows brains, initiative, assertiveness and restraint. Though when he meets her he is tongue-tied.
In the film the old woman goes to Ada, says she wants Inman to plough the field, and says that she thinks Ada can persuade him. Ada flirts openly with Inman, who is too shy to reply, but is later seen ploughing the field: which makes Ada realise how much he likes her.
It's a complete inversion of the dynamics, making the women the operators. I am quite happy with either scenario, but it is an interesting change, and I wonder why it was done? Perhaps because of the actor, or because of Minghella's requirements for the story.