Lord of the Rings is a big story with a dramatic buildup. At first it seems like the quest would unfold in a The Hobbit-like manner, with an eclectic band on a journey of adventure, but the story gets more and more serious. The contrast between the start (a birthday party) and the climax (entire world at war for survival) is astounding.
It's in hindsight where some elements near the start, which make sense for a light-hearted fantasy story, now seem out of place after the entire story is told. Hobbits and the birthday party, described in excruciating detail. Tom Bombadil, mysterious in his disconnection with the world, let alone the story. In fact, many locales and factions presented in the first book are never heard from again until well after the climax, such as the contribution of Erebor, Lothlorien and Moria in the war. Contrast this with the factions encountered in the latter two books, which all tie together with the larger story: Fangorn with Isengard with Rohan with Gondor with the Black Gate.
Therefore, I suspect that Tolkien originally intended a The Hobbit-like story, with the serious-business, geopolitical stuff coming out of serendipity. It's as if Tolkien realised how much drama would be available if he smashed all the factions together in diplomacy and war. Or he realised how many more interesting settings he could write about if he split up the fellowship and jumped the narrative between them. Mind you, this is no indictment of Tolkien's writing skills; many great writers let their characters and setting come to life and take the story in unexpected directions, and it's to his credit that they are so believable and unique, and thus could tell such a grand story.
If you accept this thesis, a few other questions about the story make more sense, like how easily the fellowship broke, or how anticlimatic the Battle of Bywater seemed.
Did Tolkien ever state whether he changed his mind about how Lord of the Rings would go? Was there a very early draft or plot outline that was different in this manner? Did Tolkien ever explain why the tone of the story changes so dramatically during the first book?