From what people on the internet tell me: everything.
IMNSHO, a lot of people forget that he was human. There's a weird deification where his words are treated as some sort of gospel. In reality, he rewrote things. He made mistakes. He edited things. He changed things over time.
Lots of stuff happens in The Hobbit because it was supposed to be a fun book for kids, LotR is just the sequel, and he had some other neat ideas. The idea that we have to treat every word like it's the gold standard for world building is just weird. Sure, Tom Bombadil could secretly be the most powerful being in Middle Earth. Or he could just be a funny stopping point along the way whose only real magical power is avoiding an editor.
