I still can't stop but think how stretched out the Hobbit films were, for an otherwise short book. That's a sin that stains forever with me. The Lord of the Rings films, do have their own flaws, but they're fairly contained and can be debated as to whether they were good or not. I have so much trouble excusing anything with the Hobbit though.
And I hope we never, ever see anyone tackle The Silmarillion. Simply because, there will be so much improvised fluff in that would-be film, that it'd make the Hobbit films blush.
One egregious example of this is the battle near the end of the book. My memory is a bit fuzzy, so please forgive if I slightly botch the details.
Bilbo has no reason to care about the fight. The audience has no reason to care. So Tolkien doesn't write the fight, and just knocks Bilbo unconscious at the start. One of the movies devotes a significant amount of runtime to the battle, despite the fact that audiences still have no reason to care. But you gotta have the big setpiece, right?