Frankenstein has many elements of a horror story. What strategies and devices does Shelley use to make the story scary?

Concerning Frankenstein being a
horror story, I always chuckle a little when I read Mary Shelley's account of how she
thought of the idea for the story, and how she wanted to create something so horrific. 
By our standards today, the novel isn't really what we think of as scary.  It certainly
doesn't belong in the category of today's slasher
movies.

Her novel is mostly one that involves horrific
ideas.  Humans creating life is creepy, and a bit horrific.  And creating life in secret
while obsessing and suffering from what one might call fits is a bit scary. 
Irresponsible science is scary (just look at the twentieth century's creation of new
weapons).  But this is a kind of somewhat sophisticated horror, a horror of
ideas.

Shelley does use some traditional methods to make
her novel scary, however. 

• Blood and gore.  We
  are certainly familiar with that being used in stories today.  Body parts abound, scars,
  the hideous appearance of the creature, the destruction of the female creature that is
  half finished.


• The killing of the innocent.  The creature
  kills a little boy.


• Plot twist.  The reader's focus is on
  Victor and the creature killing him on his wedding night, but the creature kills his
  wife instead. 

Though the novel might not be
so scary by our standards, it has actually become scarier over time.  The creation of
life was a fantastical dream at the time it was written.  Today, cloning, though not
actually the creation of life, is certainly creepy in the same way the novel
is.