Movie vs. Book?

As you may have noticed, my new Book du Jour is the Maltese Falcon, and can I just say that I'm thoroughly enjoying it? I love the description of the main character, private eye Sam Spade, who "looked rather pleasantly like a blond satan." I guess the casting execs decided that a "blond satan" wouldn't sell very well as a hero in the movie version, which starred Humphrey Bogart.

Which made me think of all the other book/movie combinations and the differences that exist between the two mediums. How many of the changes that are made when the books are turned into movies are actually necessary? Would the Wizard of Oz have been less successful if Dorothy's slippers had been made of silver instead of ruby? Or the Lord of the Rings if Frodo had been 40-ish?

But then, some movies have actually improved on the books they were based on, such as the Princess Bride (My apologies to any fans of the book, but having all the characters die or be captured at the end is just plain wrong!). Others, of course, have done the opposite and ruined the story (i.e. the Golden Compass - fabulous book, horrible movie!).

I recognize some changes are necessary, since there is indeed a significant difference in what can be done with a 2-hour movie and a 350-page book, but I'm always a little leery of going to see a movie if I've already read the book. There have been far more failures than successes as far as I'm concerned.