Undead?
Things man was not meant to know?
Tomes of ineffable alien evils?
Teens having sex being the first to go?


What says iconic horror to you?

Haunted houses
Animated objects
Serial killers
Masks
Kitchen knifes & cleavers
Screams
Tentacles
"Do I look fat in this outfit?"

Undead
The omnipresent villan
wandering off alone

To me, iconic horror is anything you know you shouldn't look at, but can't tear your eyes away from.  Nice and vague, I know.

I prefer:

Bad guys who are seriously bad. See the baby-crushers in Seventh Curse.  Watching the bad guys turn one hundred infants into Baby Goo so they can summon a demonic assassin does more to make you hate them than the previous eighty-or-so minutes of runtime combined.

Fates Worse Than Death. Kill a character and he's offscreen, finished.  Mangle or mutate him, and you have to deal with that for the rest of the story.

Random Cruelty. Cube was able to build up unbearable suspense with only two onscreen deaths by trap.  It was the randomness of those deaths that kept you from feeling "safe" about any of the characters.

Monsters you can kill. There's nothing more boring than characters fighting something they have no chance against.  You have to give people hope before you can crush it (John Carpenter's Thing used this to perfection, giving you a Thing that the characters could defeat if only they could work together to hunt it down.)

Boobs!
blood
gratuitious marijuana smoking
only 1 survivor per story

Two words...

THE THING!

That film still has the ability to make me jump!

Characters who are "baptized" by blood before they can adopt the atavistic violence they must embrace to survive.

"The Last Girl" syndrome: the girl who shuns promiscuity and other pleasures of the flesh is always the one who lives through the ordeal.

The amoral, uncaring, cosmicly powerfull, unstoppable, pile of crunchy grimy guts that would unthinkingly devour a hundred or a billion people to absorb their conciousness and torture it in order to amuse itself for a moment or an enternity, just noticed you, or your family.

What fonkaygarry said.

Also...

Breakdown of Causality - Things are not what they appear to be.  Cause and effect are no longer directly and understandably related.  "Did I ever tell you my favorite color was blue?" says Sutter Caine to the protagonist in In The Mouth of Madness, and suddenly everything is blue.  There is no rhyme or reason...  or rather, understanding that reason leads to answers which are all the more terrible for their revelation.