If a friend asks you to recommend a really good book—good writing, good characters, good story—but with no other qualifications … what would you recommend?
There's something for everyone, at least everyone who likes fiction, in this book. The world is so deep and fully realized - there are things mentioned which clearly have their own history, known to the characters, but not required for the story.
Consistency is well done throughout - Tolkien was even carefully checking the dates for the phases of the moon to make sure that the characters, separated were looking at the same moon on the same day.
The devil is in the details people say, and Tolkien put a lot of effort into making sure those details were all consistent throughout the story.
The same thing is true of the characters. They have depth, history and motivations which make them into real people who grab your interest and hold on to it until the last page is turned.
Personally, I love all the description, but I know some people have said there's too much of it. For me, it's that description that helps add the depth to the story - I can almost feel myself in Middle-Earth at times.
Tolkien knew language and how to use it, and that shows in every carefully chosen word in the book. Prose, poetry and song were all carefully written to best effect.
I could keep going for a while, but I won't. The Lord of the Rings has been my favorite book for about two decades now, and it shows.