As a novelist, Tolkien is overrated. That is not to say, however, that I dislike his novels; on the contrary, I adore them. It has to be said, though, that he was not a novelist, first and foremost, and it shows; The Lord of the Rings is overly descriptive, has serious pacing issues, and there are so many references to events in The Silmarillion, which wasn't published for 20 years after LotR, that it can be more than a little daunting.
Where he is not, and could never be overrated, is in his world-building, which has had an astonishing amount of influence on modern fiction; practically every epic fantasy novel, film, game and comic is indebted to his work. It's certainly telling that the best high fantasy films are adaptations of The Lord of the Rings. He wrote his novels not so much for the sake of telling a story, but more as a way of putting his world to paper and making it, in a sense, real; this is the reason why The Lord of the Rings can be a little tiresome to read. He created a history for Middle-earth which goes back thousands of years; he gave it gods and a detailed cosmology; he wrote creation stories for all the races, and explanations of why they are the way they are; he drew up maps of his world and gave it realistic and consistent geography; as such, the depth and complexity of his world simply cannot be overstated. It is this world-building which created the fantasy genre as we know it today, and in this regard it is impossible for his work to be overrated.
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