The differences appear to be minor.
There are small variances. The character of Audrey Wiler is removed from the story. When Billingsley gets killed by the mountain lion, David is lured to the projection booth the ghost of his dead sister. The truth of what happened at the China Mine back in 1858 is revealed to David by God via a sepia toned motion picture (even though video recording devices did not exist in 1858).
While David is presented as a godly person and perhaps one of the truly blessed, his special communal abilities are not played up. The references to the Ho Chi Minh Trail and the Vietcong Hideout are removed and he never communes with God in the tree house as he did in the book.
Instead, we find out that Johnny Marinville had an encounter with Tak back in Vietnam and that Tak blew up a bar, killing dozens of people.
The movie’s climax and end play out just as they did in the book with Mary escaping from the shed at the base of the mine and the others descending into the mine where ultimately, Johnny Marinville blows the mine, sealing Tak forever. The rest of the group escapes in the Ryder truck.
The primary difference they noted was that the book is more contemplative about the nature of God and man, while the miniseries focused more on action.