Actually, THE EXORCIST has a fair amount of Dick Smith ugly face, vomiting, head turning, crotch stabbing gore, which is probably why it wasn't mentioned.
It is one of the scariest movies ever. though. As some people have mentioned - gore by itself can be less scary and more shocking and repulsive, so perhaps you should think about "what is scary", period. Remember that movies like THE EYE were shot bloodless, then the studio stepped in with bloody reshoots to spice it up. Don't force yourself to lose scares by being too conservative.
A couple with gore, but worth studying:
JAWS (some gore, but it was all about the P.0.V. shots!)
THE GRUDGE (some gore, but mostly shocks)
I have to second a lot of what was already mentioned (relatively bloodless)
THE SIXTH SENSE
ROSEMARY'S BABY
SESSION 9
THE CHANGELING
WAIT UNTIL DARK
Some more:
HALLOWEEN was relatively bloodless. Study that!
INVASION OF THE BODY SNATCHERS (paranoia)
WHEN A STRANGER CALLS
THE DEVIL'S BACKBONE
THE VANISHING
DON'T LOOK NOW
MOTHMAN PROPHECIES
ARACHNOPHOBIA
AMITYVILLE HORROR
SIGNS
SALEM'S LOT
THE BIRDS
VILLAGE OF THE DAMNED
OPEN WATER
Totally agree about THE TEXAS CHAINSAW MASSACRE, but it and PSYCHO are older movies. It is interesting to look at any old horror film, to see how they handled a rough scene. THE THING (the original) is an interesting example, as is the movie that inspired ALIEN, IT! THE TERROR FROM BEYOND SPACE.
FRANKENSTEIN, THE WOLFMAN, THE MUMMY are all fairly interesting. THE WOLFMAN's metamorphosis was scary to me, because that meant he was going to become a beast and do something bad! The slow metamorphosis of Jeff Goldblum into THE FLY was very unnerving.
Scary things:
Noises - scrapes, breaths, bumps, screams, growls.
Music - John Carpenter made the term "Stinger" famous - a shocking, stinging sound, often used for "false scares".
The night - howls in the darkness, something outside your window.
Obscured visuals - use of light, shadow, darkness and fog.
Insects - spiders, scorpions, centipedes, tarantulas.
Beasts - sharks, crocodiles, snakes, bears.
People - large men, little children, cloaked figures.
Water - oceans, murky swamps, lakes.
Big houses/buildings - nothing scarier than the hospital ward scene in EXORCIST 3: LEGION.
Precipices - cliff sides, deep dark wells, elevator shafts.
Bondage - being tied to a pole, a chair, in a basement.
Basements/attics - dark, dusty places.
Caves - I refer you to THE DESCENT.
Staircases/stairwells
Parking garages
Unbalanced people - happy one minute, manaical the next.
Sharp objects - knives, stakes, razors (the mere presence of which can be suggestive).
Solitude - alone and far from people or help.
Smothering/trapped - buried alive in THE SERPENT AND THE RAINBOW.
Dead return to life
Apocalypse/end of the world - This whole "2012" (not just the movie) intrigue is off the map.
Revelations - THE OMEN
Someone behind you - even worse when you can see it, but the main characer can't.
Clowns - yes, clowns, dolls and puppets.
Dreams/nightmares - Ever see A NIGHTMARE ON ELM STREET?
Hell - demons, Satan, fire.
Burning
Drowning - stuck in a sinking car, with a small pocket of air.
Disease
Religious cults
Cannibal rednecks
Stalker boy/girlfriends - FATAL ATTRACTION, anybody?
Organ stealers
Laboratories
That's a start. Now, start combining some of those, like bonded in a burning lab, with a razor wielding stalker girlfriend, who is a clown?!