Its true, when they take a comic book and make a movie out of it they tend to do the same thing they do when they turn a book into a movie. The story gets changed and warped. Its generally better to just hope they do something good with it instead of hoping for a movie that is true to the book/comic.
This is partly because of how they have to do it. Sometimes they don't have the freedom to do a true conversion from one format to another but there is a bigger problem. When ever you read in a book that someone thought something or felt something, that will not transfer to a movie correctly unless exaggerated.
A book, even if its a comic book can easily tell you what the characters feel and think. This is quite the boon for the format as it lets you have incredibly deep characters that you can get across to the reader without to much effort. On the other hand books have a hard time with action scenes. Even comics which have a more visual format can not truly impart actions with out exaggeration. Movies though are the exact opposite of this as they can easily with no effort at all show motion and action, even the small things like having a character quirk his eyebrow at something someone said.
Basically what this means is that the two formats are dynamically opposed in what they can tell the viewer. If you want a person shown who is deep in thought a book would be better for this because in a movie unless they do the cheesy inner monologue thing you basically just have a guy sitting quietly by himself. Now an action packed fight is the movies forte. While a book could take pages and pages trying to describe a short fight the movie just shows you it.
Comic books are more of a middle ground as you can show and tell but overall they are closer to books then movies. This means that they should technically transfer over to movies easier then just a book but there is one thing that stops the smooth transition.
Comic book fans are into canon. Now you may wounder about this as comic books get quite a number of rewrites and such. Trying to find out what is truly canon in a popular comic book series is quite hard. This is doubly true if the story is written by several people over its years. My personal opinion is that this is why the fans care so much. They love the story but the authors keep trying to change it on them. If you say love the story to Lord of the Rings and some author came along and completely rewrote it and said his story was now the real story and the old one never happened you would definitely care. This is something comic book fans have to actually deal with.
From what I have seen of the movies done from comics they tend to get a number of things right, more so then a good number of movies based off books ever did. The problem is that the fans of comic books have been sensitized to this kind of thing over the years and are quite willing to complain about it. Even worse because the comics actually showed you what the people looked like anything that's changed at all about the character will easily be noted and despised. In a book you say a person has blond hair and so long as the movie uses someone with blondish hair your fine, in a comic if the character is drawn with blindingly blond hair if you even just have a regular blond play the role people will notice.
Overall the movies tend to be okay but because of the general inability of Hollywood to make something into a movie without perverting it and the semi visual format of comic books combines with a fan's love of their personal view of how canon should go to cause quite a bit of complaining and arguing to go on. I will admit though as the Spider-man's movies have shown, the more sequels they do the worse it gets or as emo-spidey would say, "why wont the pain stop".