1. Lillian is practically a saint, the worst thing said about her is "She never fit in here, and that's why she left." There is no mention of her sleeping with Lugene Brooks (who is a woman in the film), Nick Wickham, or LaRiviere. Not to mention her affair with her lawyer is never brought up. When Lillian tells Wade that she is disgusted by him going to a new low, she is only referring to the custody lawsuit.
2. Wade is crazy the entire movie. Honestly, it totally misses the whole slowly descending into madness aspect. He seems to be a bit insane the entire movie.
3. The truck never goes under the ice, Jack just shoots the tires out.
4. Jack's death is in the epilogue, with a whole 5 seconds spent on the entire scene.
5. Most of the post-tooth pulling events don't occur. Wade never sleeps with Hettie, never talks to Asa Brown. Just pulls the tooth, gets his daughter, grabs Wickham (a lot more forcefully), finds Margie moving out, throws his daughter, then talks to his father about love, before killing him.
6. Wade blows up his father's truck when he's burning down the barn, there's no explanation how he gets up to the mountain to kill Jack.
7. The story that Wade confuses with Rolfe is not of getting beaten for the bathing accident. Rather it is of a wood chopping story that somewhat combines the barn story and the bath story. Overall, the father seems much less violent, and more incoherent than anything else.
8. Wade is very surprised and remorseful that he killed his father... in the novel he seemed sentimental, but definitely not remorseful.
So, the film representation does do a decently good job of telling the story, but the novel, naturally gives you a much better insight to Wade. His emotions, his frustrations, his downward spiral into homicidal madness. The movie glosses over these, which is unfortunate for people who view the movie and not read the book.