The really great thing about The Lord of the Rings is its realism. Tolkien may build a vast world, but he's concerned with the tiny little domestic details in it. He doesn't start out with, "Thousands of years ago, there was an epic war and the Ring was lost." He starts with an eccentric old man having a birthday party. (Okay, he starts with a long explanation of hobbits, but that's a different problem. I'm focusing on the story itself). The epic parts are slowly added into the world, bit by bit, when they're relevant, and they fit in seamlessly because of how thoroughly he's established the ordinary details of Shire life--houses and petty family squabbles and gardeners and who owns that field kind of stuff.
By doing that, he's showing us that this is what matters. Home and family and community. The adventure is just extra, taken up unwillingly to protect all the important stuff. The Fellowship starts with five best friends who are willing to help one of the group move to a new house. (And four guys who totally respect the contributions of the one guy who doesn't want to go on the larger adventure). They're not a Fantasy Traveling Party. They're just friends going off to battle. (It's very easy to read the hobbits as Edwardian young men going off to WWI, because it's built out of that setting.) Their ordinary friendship is important, and their ordinary lives are important. They know every ditch and field and river in the Shire, and when they run into danger they have the comforts of home to help them. Baths at Frodo's house. Farmer Maggot's hospitality. Even, yes, Tom Bombadil, who is the perfect escalation of this concept--now even the helpful, comforting things come with a side of the otherworldly and unfamliar. There's an escalating sense of danger that's willing to take it slow. It's not all danger, but the tension is building because we know this is only the small stuff, so what dangers lurk out in the wider world?
It's more like a portal fantasy or an intrusive fantasy than a pure secondary world fantasy. It's about people leaving the ordinary world, or about the fantastical intruding on the everyday. And part of that's because he's not writing a genre story. Writers who came after him are writing stories set within the genre: elves and dwarves and epic battles with supernatural overlords are the starting point, and they build from there. Tolkien started with the real world, and appreciated the real world as much as any of his innovations, and it makes his story so much more grounded and compassionate and real.
