I've been a fan of The Lord of the Rings ever since I first read Tolkien's books aged 8. I would go on to re-read them several times over the next few years. I first watched the film adaptations as a teenager, and to this day they are some of my favourite films. Like the books, I have revisited them many times.
In the past, I had plenty of criticism¹ of the films as adaptations, mostly regarding details being changed or omitted. I have not retained most of these opinions, as I later realised that the point of an adaptation is not necessarily to replicate every single detail of the original (which was what I expected from film adaptations when I was younger). It is more important to preserve the ideas and messages of the source material. Hence, some of my criticism of the film trilogy endures, notably of the ending.
The scouring of the Shire is one of my favourite parts of the LOTR books. Just as Frodo and his friends think that they've won and their journey is over, they have to fight one last battle, this time to save their homeland. Not even the remote Shire is safe from the effects of momentous events in wider Middle-Earth, and the hobbits, portrayed as insular and willfully unaware of the outside world, are forced to acknowledge this. They also get the chance to prove their mettle fighting evil and protecting what they love.
This ending provides closure; not only does it explain what happened to Saruman and Wormtongue and turn Galadriel's conversation with Frodo into foreshadowing, it reinforces the anti-isolationist themes present throughout the books. Frodo leaves his provincial homeland to defeat Sauron; a fellowship is formed of the races of Middle-Earth; the Ents, who previously insisted on being 'on no one's side', help Théoden's men at Helm's Deep and rise up against Saruman; the riders of Rohan come to the aid of Gondor; and so on. It is also a logical conclusion to a story that is anti-escapist² from the beginning (Frodo must take the ring out of the Shire himself).
The scouring of the Shire is absent from the adaptation, and thus the film trilogy lacks this closure. When Frodo, Sam, Merry and Pippin return to the Shire in the films, nothing has changed. The rest of the hobbits are living their lives as they always did, unaffected by their wider surroundings, despite the monumental upheaval that just took place in Middle-Earth. Their insularity continues unchecked. As for Saruman and Wormtongue, they just disappeared, I guess. The idea the film conveys is that Frodo and the Fellowship saved Middle-Earth so that the hobbits could continue living the way they do. It's a much less satisfying ending, and undermines a key message of Tolkien's work.
I found out about the LOTR extended editions years after watching the theatrical versions for the first time. I was excited at the prospect of being able to watch things I felt were missing from the theatrical versions. To my dismay, I discovered that the extended editions still omit the scouring of the Shire, while adding scenes that don't take place in the books. For instance, Saruman is killed at Isengard, and the Mouth of Sauron is portrayed as a monster rather than a man, which completely misses the point of his inclusion in the story — to show how humans too can be slaves to Sauron. I still haven't watched the extended editions; I'm much less enthusiastic about it than I used to be.
¹My enjoyment of the film trilogy was never affected that much by its (as perceived by me) flaws.
²The Shire in the books can be interpreted as an allegory for living in willful ignorance. Sooner or later, your surroundings' issues will come to affect you. I'm reminded of the legions of people who believe that they would be better off living in cabins in the woods, or as animals ('reject humanity, return to monke'), because they want to escape society's problems to be happy. I have little respect for this viewpoint; it's a total cop-out. Animals and people in remote locations are impacted by society's side effects (e.g. climate change) all the time. I can think of few things more cowardly than knowing a problem exists, and then instead of doing something about it, running away for it to inevitably come after you.