Help
The three points about his world building that are impossible to reproduce are:
1) he was deliberately trying to create mythology
2) he followed the rules of the world he created to a fault
3) and he cared just as much about language as he cared about the story
Unfortuantely I have none of my sources with me here in Japan, but Tolkien wrote that he always felt that England lacked a pre-Christian mythology of its own, a la Ireland, Northern or Eastern Europe. Thus one aspect of Middle Earth was to craft a framework for what could have been English mythology. LOTR becomes one fleshed out story in that mythology. Because of this, one important detail of Tolkien's works that differs from other authors is that Middle Earth is supposed to connect to our world. (After the fall of Numenor, the heretofore flat world was made round, the Undying Lands were removed and made inaccessible to mortal men, and the geography of the Shire and Lindon roughly ends up lining up with that of England on our modern globe.) One of my favorite stories of Tolkien is that he regretted deeply that the names of the Dwarves in the Hobbit (which were pubished and therefore finalized) almost all come from the Norse Edda. After many many years of puzzling on the issue, he realized that the solution to the problem was that the Scandanavian languages which produced those names took them from translations of stories that were originally told in the language of the Dale.
Another interesting element is the metatexual device that Tolkien uses to bring us the LOTR. It is actually supposed to be a translation into English of the book of Westmarch, (which contains Bilbo's There and Back Again, and Frodo's orginal writings, but was then edited and presumably altered by Sam's descendants) from the Westron tongue. Unlike some authors who might tack on such a frame story, Tolkien's other, unfinished works don't suggest that he would be the type to take such a device lightly. In writing the Lord of the Rings, he had to consider how the story in all its shades of gray (a story like tMBotF, perhaps) would be simplified and changed through the filters of hobbit and later human interpretation. An example of where this has relevance is in defining a villian like Sauron: the story arc of his character notwithstanding, Sauron is very much an unfathomable evil to the hobbits as opposed to how in other historical fragments his actions on the human scale are more understandable. (I'm not articulating this point very well, but I need to sleep so moving on...)
Finally, as a linguist, Tolkien spent an incredible amount of time working with the languages of Middle Earth--as the key aspect of his world building. Changes in the names and relationships of characters had to match the proper linguistic principles, which sometimes came first. Erikson might decide that there have been x number of Jaghut wars over the last 100,000 years as a detail of his world, and then arbitrarily decide that 4,000 years have passed since the 33rd war. In some cases, however, Tolkien would "discover" that a certain vowel shift took place between high elven language Quenta and the related language Sindarin, and then build the history based on how long it would have taken for those linguistic changes to develop. If thereafter he wanted to change the name of a Sindarin lord, he might need to reevaluate his timeline if it violated the rules. Likewise if a word that didn't fit came into a language, there had to be some reason for it in the story. Apparently the Tolkien estates still have rooms full of text containing poems, songs and histories of Middle Earth written in various languages of its people -- that pretty much only Tolkien could have understood.
