While the colorful and vividly grotesque descriptions of various nightmares, alien cities and otherworldly horrors are indeed powerful, it loses a bit of its punch when we see the narrator describe almost every non-Caucasian in the exact same way.
There's this great buildup describing Cthulhu, the grotesque Eldritch Horror... that "flopped" down the city stairs in pursuit of the sailors. Yes, flopped. While Cthulhu is terrifying, the word choice evokes laughter.
Also, the great Cthulhu was literally thwarted by a boat to the head. ...Admittedly this is with a ton of buildup and it only works for about ten seconds before he starts regenerating.
The Mountains of Illinois: R'lyeh's location as given by Lovecraft (47°9′S 126°43′W) is just above the Pacific's abyssal plain, many miles away from the oceanic ridge where volcanic islands may rise and fall.It has been chosen for the closeness to the Pole of Inaccessibility - the point farthest from any lands.
Values Dissonance: The non-white cultists are described in terms that make them sound more like savage animals than human beings. Lovecraft's depiction of the cultists isn't much different than, say, D.W. Griffith's perception of "The Negro."