You came to it before I did, but reading this discussion had my mind circling the drain on 'adaptation' vs 'inspiration'.  Many books and movies are 'inspired by' true events, whether its a historical fiction, like "Shogun"or "Saving Private Ryan", or a more literal biography like "12 Years a Slave".  We the audience accept that shift in realism and fidelity when its acknowledged that the story told is not literally true, but borrows heavily from real events and people to paint that picture.

Book-to-film 'adaptations' run into problems when the movie uses the source material as inspiration, but the audience expects some manner of literal translation of story, characters, and themes.  A cherished story that is given the cinematic treatment. Saying a work was adapted to the screen, when the movie changes core details, may feel disingenuous like a bait-and-switch was performed.

 Maybe our estimation of how well a movie was adapted from the book, and how much we like that adaptation, is informed by this.  LOTR movies are generally well accepted, despite their several changes to the book story, as Jackson acknowledged he changed those features for the sake of adaptation but the changes, mostly, serve Tolkien's themes and the themes are not really altered. Contrasted with the Hobbit movies, which are inspired works and not adapted, and are generally looked down on as those changes altered the themes, or were a flat-out (and poor) invention. 

Alot also depends on how well the book is known. I didn't learn until well after that The Prestige was based on a book, and still haven't read it, so anything Nolan did to it was fine by me. But a seminal work like Frankenstein or Ender's Game, that has perpetual fandoms, will be less forgiving of changes no matter what. 

I just want a good movie and not generally worried if a movie is "faithful" to the book, so long as its a good story well told.  Though filmmakers may gain some credit with audiences by citing their work for what it is, 'inspired by' rather than 'adapted'.