You've got to respect the man for his attention to detail. To the outsider it might seem as if it were near insanity levels (fully creating multiple language families for instance https://en.wikipedia.org/wiki/Languages_constructed_by_J._R....). But you feel the histories and mythology behind the languages in the works. There's just an amazing vitality and realism in his universe that I've personally never felt in another work of fantasy as good as those other books might be.
I read a biography about him many years ago, and if I remember correctly he wrote the stories for the languages not the other way around, since he discovered that his languages needed a mythology and history in order to be realistic.