Changing the human-looking vampire of the book was the key to this miniseries' success. (Possible Spoilers)
I just finished the book again after a very long time. If they had used the human-like book version of Mr. Barlow that Stephen King created, this would have been laughable, even back in 1979. Thankfully, the makers of the miniseries used the much creepier look of Nosferatu for Barlow. This resulted in a classic horror film that stands up even now, 40 years later.
And the book also made a huge mistake in moving the Vampire's lair to the boarding house. All this buildup about the Marston House all through the novel, and then halfway through the Priest blesses it and King drops the house entirely. Another reason the show was 100% better than the book.
King came up with a good basic idea, but the execution in his book was lame. This and The Shining (1980) and also Carrie (1976) prove that King's ideas need a lot of work and outside help to be truly great.