A movie interpretation is rarely the same as that of the original media, but picking up the important things of the story and then the director uses it to tell his/her version of it. Doesn't necessarily make the movie bad - it's just not exactly the same as the original media. If people expect it to be the same, then they are expecting too much.
There are of course some cases where the movie DO turn out to be bad. E.g. the Last Airbender movie... though it was beautifully made, they totally dropped the ball on the characters' personalities which is what makes the anime so good, in my opinion. Oh, and Tekken too... but then again, movies based on fighting games rarely turn out well...