I agree that comic books and movies are two different animals (though an argument can be made that cinema was what spawned what we now think of as comic books, and the two have a lot more similarities than differences). If changes need to be made for a book to translate well up on screen, no problem. But there comes a point when you change a character so much that it might as well just be a different character.
Not a comic book example, but let's take the movie I, Robot. It wasn't a bad movie. In fact, for a summer blockbuster, it was actually a pretty good movie. But it neither lived up to, nor had any real similarities to it title material. If I, Robot had been called something else, I bet critics and audiences who had read the book would have looked at it a lot more favorably.
That said, change is good. Change is often necessary. But don't change things so much that it just becomes something completely different. I'm not saying this is what happened with Constantine (though I still consider a Sting-inspired Brit to Keanu Reeves to be a big change), by the way, but it has happened with other things