When I read it again last year I was amazed at how easy it was to put down at any point. There were two basic causes.
The first is that - in accord with accepted literary practice of the day - Tolkien uses the passive voice. Today people are told to use the active voice. This difference in sentence construction changes the verbal pacing and immediacy of the entire series.
The secod, and related, cause is that he told it as a series of small self-contained stories. Modern novels use the cheap trick of always having hanging story lines. Whenever you switch from one scene to another, you always have some sort of cliff-hanger.
Neither goes over well with people whose attention span has been trained by flipping channels on TV. However fast paced action will simply never touch Tolkien for the richness of mood and story. His is a world you need to visit, sit back, and enjoy for a while.
Cheers,
Ben