Tolkien wrote his classic children's book The Hobbit in the 1930s. In it, Bilbo the hobbit goes on an adventurous journey. Along the way he finds a magic ring which will turn the wearer invisible. He escapes from goblins, fights with giant spiders and even talks with a dragon.
The Hobbit was very successful. The publishers pressed Tolkien for a sequel. While he wanted to publish his stories about the Elves warring with Morgoth, the publishers wanted 'more hobbits' so he came up with the idea of The Lord of the Rings. Bilbo's ring was now moved to centre stage and was much more than just a tool. It carried the spirit of Sauron, the Dark Lord, so it corrupted its wearer.
This didn't quite match the references to it in The Hobbit, so he made some changes to The Hobbit and these were published as the Second Edition in 1951. Every edition since then has included this revised text. First editions of The Hobbit with the original text are available but rare and worth thousands (a copy in good condition could fetch €30,0001) so up to now there wasn't much chance of an interested reader getting hold of one.
On 31 May 2018, the publishers produced a facsimile edition - an exact photographic copy of one of the original first editions. The only difference is the publication page, which includes up-to-date publication information. This facsimile makes the original text available for a relatively modest sum, although it'll still set you back about €40.
I've been reading through my copy without taking detailed notes, and have noticed the following differences:
The dustcover features a runic inscription around the outside which describes the book. In later editions, this inscription is at the top and bottom of the title page.
The foreword explaining about the runes and about the use of the words 'dwarves' and 'orc' does not appear.
There are a couple of typos which were corrected in later editions: "nay" for "any", "momnet" for "moment". There's also a typo in the blurb on the dustcover: "Dodgeson" for "Dodgson". This was corrected by hand on every one of the 1,500 first editions - this facsimile has the 'e' inked out so that it appears as a small black block.
Hobbits are described as smaller than dwarves but much bigger than Lilliputians. This reference to an imaginary creature from another author's works obviously didn't sit well with Tolkien. He changed it to say that they are about half as big as humans2.
The Took family are rumoured to have fairy blood. In the later edition this rumour is reported but immediately dismissed as absurd. Tolkien's idea of fairies had become more structured - the 'fairies' of his world were angelic beings, and too important to marry a mere hobbit. He dropped the term fairy and when necessary referred to them as Maiar.
Gandalf is described as a little old man. In later editions he was quite tall, though not as tall as Saruman or Aragorn.
Gandalf uses the sharp point on the end of his staff to scratch a mark on Bilbo's door, as he did in the later edition, but this is the only reference in the book to his staff. Everywhere else in the book he has a wand which he uses for doing magic. In the second edition the wand is removed. He is explicitly given a staff in the very first description of him, and he uses his staff in many places where the wand had been used.
When Bilbo is talking about Gandalf coming to the Shire in the past and persuading young hobbits to go off on adventures, one of the things they did was 'crossing over to the other side'. This is not explained any further but sounds suspiciously like going to the undying land of Valinor where the High Elves live. In later books, Valinor was unreachable except by the Elves themselves.
Elrond uses the term Gnomes to refer to the High Elves of Gondolin. The word 'gnome' does in fact originally mean one with immense knowledge, but Tolkien was persuaded that it didn't really suggest the idea of beauty and respectability that he thought, so in later editions Elrond says 'the High Elves of the West, my kin'.
The biggest difference from later editions is the characters of both the ring and Gollum, who are intertwined. Bilbo's ring is a magic ring, but nothing more than that. Gollum uses the ring, but he is not enslaved to it. As a result, the whole encounter between Bilbo and Gollum has quite a different tone. Although Gollum is interested in eating Bilbo, he is a far more decent character than in the later books. He has the riddle competition with Bilbo because he is bored and not particularly hungry. He promises to give Bilbo a present of the ring if Bilbo wins, and explains to him how the ring works - such a thing would be unthinkable in the later books. When he can't find the ring (he doesn't know that Bilbo has already found it), he agrees instead to show Bilbo the way out of the mountain and the two of them walk along the passages together talking.
There's a reference to policemen early in the book. This makes the Shire seem far too like the normal world, so it was removed. There are policemen in the Shire in The Lord of the Rings, but they are called 'Shiriffs' to retain the ancient feel of the place.
White Council - in the later books it was clear that the White Council included the wizards and the chief Elves, including Galadriel and Elrond. In the First Edition, Gandalf explains to Elrond what took place at the White Council, suggesting that is was wizards only.