One of the things that irks me personally, though I admit maybe only really diehard Tolkien fans would notice it, let alone be bothered by it, is the disappearance of the 17 years between Bilbo's departure from the Shire and Frodo's departure. While in reality it makes little difference to the movie narrative of the original trilogy, it allows for a massive plot fuckup (in my opinion, which I know is probably unpopular didn't this being overwhelongly nitpicky) at the end of the Hobbit films.
Near the conclusion of the Hobbit films, Thranduil enigmatically tells Legolas to go pursue Aragorn, only hinting at his importance, and only calling him a ranger named Strider. In real Tolkien history, at the time of the Battle of Five Armies, Aragorn was still a child living in Rivendell, and only Elrond knew of his lineage. As such, he had not yet left to become a Ranger, and no one could have known of his actual importance yet, certainly not Thranduil.
Without those 17 years, it is suddenly possible for Thranduil to potentially possibly know, although if memory serves me correctly, Legolas had never met or known if Aragorn prior to the Council of Elrond, which is another hole.
Yes I'm aware this is nitpicky and that the last majority of moviegoers won't know or care, and that it allows for some sort of continuity between the movies. But also, Legolas wasn't a named part of the Hobbit to begin with, so in the end the whole thing breaks down from a purist's viewpoint.