Answer:
I believe all of the reasons mentioned make people disappointed:
The actors do not look like the characters that readers imagined.
The story lines have been shortened, and important details have been left out.
The movie's setting does not look like the setting that readers imagined.
The story lines have been changed to appeal to the public.
Step-by-step explanation:
Tom Hollander, the actor who portrayed Mr. Collins in the movie version of "Pride and Prejudice", said in a bonus feature that readers of the book often think they own the story. It is very hard to please them, and they often complain about the choice of actors, saying that they do not look like the characters at all.
One of the most common complaints of people who have read the book before watching a movie is that several details have been left out. Even though readers know that a one-hour-and-a-half movie does not have enough time and "room" to present all the information contained in a 250-page book, they feel as if those details would have made a difference. The story feels incomplete to them.
It is also common for readers to come up with their own images of the setting, each person taking the author's description and developing it in a personal way. I, for instance, imagined the Diagon Alley from the Harry Potter saga very differently than the one presented in the movies.
Finally, movies need to be profitable to those who produce them. That means adapting the story lines to make them more "sellable". The movie "Troy", for instance, greatly changes the ending originally given in the epic poem "The Iliad". In the movie, Paris and Helen stay together; in the poem, Helen is ashamed that Paris is such a coward and chooses to leave him and go back home with her husband.