Last night I finished reading True Grit by Charles Portis. What an excellent book! I can't say that it changes my opinion of either film immensely, because both attempt valiantly to be faithful to the text. I will say that the Coen brothers' version is a much better adaptation, however. They are careful with more details and create a mood more similar to the one Portis creates. The Oscar nomination is well-deserved.
So what are we missing by just watching the film? A few things... Like the article John posted mentions, Mattie is telling the story as an older adult, so there are a number of asides where she comments on politics and religion, or simply states her very matter-of-fact opinions on various topics. We also get to see more of Mattie the accountant in a couple of scenes axed from both screenplays: one where she helps Rooster with some vouchers the first time she goes to his house, and another where she signs a bunch of stolen bank notes for Lucky Ned Pepper. The transcript for the trial in which Mattie first sees Rooster is longer, and when they are waiting for Ned's gang to arrive at the shack by the river, we learn more of Rooster's backstory. It was a lot of fun to have things like that fleshed out.
Conversely, the original adds the scene at the beginning, where Mattie interacts with her father, and the scene at the end where Mattie invites Rooster to her family plot. The Coen brothers, meanwhile, added the gross-out tongue scene between LaBoef and Rooster, as well as the lisp that LaBoef carries for the rest of the film. As always, a director must put his unique stamp on a project, and the Coen brothers, for all their loyalty to the novel, couldn't resist the temptation to make the film uniquely theirs.
It is interesting to see how Mattie's initial motivation for wanting to kill Chaney is revenge, but as the story wears on and the experience wears on Mattie, I think more and more she just wants to get the whole thing over and get back to her normal life. Except that she can't, once it's over, because the experience has permanently changed her, not just physically but emotionally and mentally. I think it does underscore John's point about revenge, nevertheless. It was revenge that drove her to join Rooster and LaBoef in the first place, and even if her motivations had shifted some, she started a machine in motion that she couldn't stop once it got going.