I think pacing and atmosphere are relative in terms of the characters and intrigue that's originally set up.
If the book really starts dragging, but i am completely invested in a character- I will still follow it through to the end (although I might peek; oh the shame!) <img src="/images/graemlins/blush.gif" alt="" />
Atmosphere; I think that more or less depends on your own imagination. I read authors like Tom Clancy and think "Get on with it! I don't have to be told what every single bolt in the sub looked like." Sometimes being given so much information is like being spoonfed, its annoying. IMHO
It has to be scary and also disgusting. For me when I am scared but not disgusting it is just suspense not horror. Without gore then it cannot be horror (for me anyway but since the word of that genre is horror then im sure im right in the description of what horror is supposed to be).