I think one of the important points when dealing with a film adaptation of a book is that we are going to be dealing with a director's vision of the film. This is a beauty of the medium of books - even though the text is the same, all of us visualise the events, locations, the characters' appearance etc. in a very different way. Watching a film adaptation therefore could certainly feel very different because we're looking at someone else's visual interpretation of the book. There is inevitably going to be a mark of the director's personal style and creative changes left there.
Another point is that there are portions of books that might not translate well onto the screen, and need to be changed suitably. For example long speeches or conversations between characters, while enjoyable to read, would drag on and bore the audience in the film. There is also the issue of pacing which may require some elements to be altered. So in my opinion, making a film which stays a true as possible to the book while making the necessary changes and remaining strong as a standalone film, is a good book adaptation.
By far my favourite series films based on books is the LOTR trilogy. They take significant liberty from parts of the books, but not only do they retain the heart of the books and stay as close as possible to the main plot despite all changes, but the characters and events/battles are really fleshed out well, helping the films to be stronger on their own too so the audience doesn't necessarily need to be familiar with the books to understand the films. This is something where the Harry Potter films kinda falter at, because although they're mostly well made and enjoyable, they miss out important details and intricacies, leaving large plot holes. It's understandable to some extent because the HP books are much more detailed and it's quite hard to fit it all in 2.5 hours. Although David Yates' last 2 films suffer from editing problems, I think he's mostly done a very good job condensing the long books into shorter and enjoyable films, but sadly leaving quite some necessary details unexplained.
I also wasn't too fond of the adaptations of Dan Brown's books either (except for Hans Zimmer's amazing music scores), especially Angels & Demons which I felt dumbed down a lot of the raw dread and suspense I felt while reading the book. I personally prefer the filmmakers coming out with original concepts, since that's where I feel they hit closer to the bullseye more often. But even books can be competently adapted properly, as long as the points mentioned above are kept in mind.
