One hundred percent agreed. That's why Anne of Green Gables (and the second series, which is equally excellent) remains a first-rate adaptation.

Other examples of this principle include David Lean's peerless 1946 adaptation of Great Expectations (one of the greatest films ever made, in my not remotely humble opinion). It trims much from the novel and does indeed change the ending, but Lean remains completely faithful to the spirit, and emphasises what is cinematic, making the film feel definitive in its own right. The same is also true for Peter Jackson's take on The Lord of the Rings.

I'd add that there are films that are considerably superior to their source material - Planet of the Apes, The Godfather, Jaws, and The Mist leap to mind. On top of that, there are some films that radically reinvent the source material, following the second great rule of adaptation: You are free to change anything and everything for one reason only: To replace it with something just as good in its own right, or even better. Example: The Iron Giant. Ted Hughes's source novel The Iron Man bears no resemblance to Brad Bird's 1999 animated masterpiece, except the bit when the giant eats scrap metal. Other than that, it's a completely different story. But it's an absolutely brilliant story nonetheless. :)