Yeah, eventhough I have to admit that Marvel pretty much follows the original comics as good as possible. If you take a closer look on, for example, Iron Man 1 or the X-men saga you can clearly see, that they tried their best to stay to that given line. But another general problem is: a movie, probably a blockbuster with alot of common known actors, wants to address as many people as possible. Meaning they don't only wanna adress a bunch of people who already know the original story and want to see how it's delivered within a movie, but also many people who have literally no idea what the whole movie will be about. I don't know if it's good or bad, since I can somehow understand that kind of thinking. Same can be seen in a very drastic way with "The Fantastic Four", as you already mentioned. As a movie itself it wasn't that bad. Ok, it wasn't good neither, but you know what I mean. But as soon as you know the original storyline, the whole given movie about them is rubbish.
Nevertheless I like movies alot and could go on for hours, just talking about a single movie by, for example, Quentin Tarantino (I'm a big fan of his work :P), which leads me to the idea, that we could or even should start a new thread just about movies and what we think of them.