Action Survivor: Lance in the sequel. Surviving for nine years in the hospital takes a toll on the sanity, but is impressive nonetheless.
Afraid of Blood: Nobody wants to be near the bathtub of blood in the bathroom (except Matt but he's insane at this point) so naturally everyone wants to leave. Matt won't get away from the tub so T.C. goes over to talk some sense into him. A pale human-looking thing explodes out and drags T.C. into the tub. The group tips the tub over to save him, only to find he isn't even in there...
Alien Geometries: The hospital itself. When the team decides to bash the front door open after a few hours, they find that instead of leading outside, it just leads to another corridor. When someone suggests that they just got the layout mixed up, Sasha finds the same markings on the door as the entrance door did— proving that it WAS the same door. A staircase to the roof ends with a wall, and corridors changing as they're walking through it. Time itself is warped inside the hospital; the camera timer shows the minutes passing faster and faster, and near the end of the movie time is moving so fast the numbers are just flashing by incomprehensibly.
Always Night: One of the characters realizes that the sun should have risen by 7:30am, but it's still pitch black outside. Later on, Lance checks the time again— it's 1PM, it's still pitch black. Even later again, the time is 8PM and there has been no sign of sunlight. It's also implied the hospital has it's own dimension.
Attack of the Monster Appendage: At one point the group is attacked by a series of black arms reaching out of the wall, and earlier in the movie Sasha is grabbed by an arm that busts through the glass of a door she's standing near, but the viewer never sees who these arms belong to, if anyone at all.
Black Magic: The ending strongly implies that Dr. Friedkin employed the use of such magic in his medical experiments, and may even be the reason for the hospital itself apparently existing in another dimension.
Darkness Equals Death: Any character moving around on their own or out of shot or view is effectively doomed.
Death by Falling Over: Matt, in his insane state, looks down the elevator shaft with the camera. The camera shows him falling to his doom, and the two remaining members both rush over to find him dead having fallen over four floors to the ground and hitting solid concrete.
It works better than other movies, though, as the characters all behave realistically, and it helps the audience empathize more strongly with their situation - a key element in a movie built on Nothing Is Scarier.
Downer Ending: The sole survivor, Lance, is shown having been lobotomized in the last scene.
Eldritch Location: The hospital. There's an increasingly strong implication that it's not even in our own dimension.
Endless Corridor: The service tunnels are supposed to have turn-offs going to various buildings, and be only about a quarter-mile long. Towards the end, there's nothing but a single, long corridor that the protagonists walk along for days.
The End... Or Is It??: The bodies of the crew are never stated to be found, and it's uncertain if the crew has become a part of the dead walking in the hospital or are gone, but it's certain that whatever is in the hospital is still there.
Everybody's Dead, Dave: One by one the characters die off, until even Lance is lobotomized and we never find out what happened to him when they found the camera equipment.
Face-Revealing Turn: When everyone finds the bathroom, a woman is standing facing a corner. While she turns her head she appears fairly normal, but after just a few seconds her face distorts horribly as she lets out an inhuman scream.
Foreshadowing: When Lance, Matt, and Sasha are headed to the elevators, they pass by a room that a loud roar emits from, but they continue. Lance later has to return to this same room, and finds out what it was.
A lot of scenes in the movie are like this.
T.C. is attacked and pulled into a tub of blood by some humanoid entity, but vanishes in the tub like he disappeared. The tub is the same tub Lance had been telling us earlier was used by a patient to slit her wrists and die in when the hospital was open.
The window from the beginning of the movie that Kenny talked about did open with Mat in the room.
Dr. Friedkin is talked about for a good several minutes at the beginning of the movie, and Mat references that "he" can help them get better and leave the hospital. His ghost shows up later in the fabled operating room.
At one point the crew wakes up and realizes Sasha's back has "HELLO" scratched deep into it - a Call Back since she had been the one trying to contact them using EVP earlier. After this, the ghosts begin starting up.
When the search begins, they run into a room with a bunch of rats in it and are grossed out. The same door leads to Dr. Friedkins operating room later.
Lance explains that "intelligent hauntings", while rare, involve spirits interacting, and that people have been pushed, scratched and even thrown. TC gets pushed down the stairs, Sasha wakes up with a message scratched on her back, and Houston is thrown across a hallway via an explosion.
For the Evulz: The ghosts surely have a lot of fun in doing all their stuff to the visitors.
Hope Spot: The crew finds a door with an EXIT sign... but it opens on yet another hospital corridor.
Prior to that, T.C. and Lance use a gurney as a battering ram to bust open the front doors of the hospital... but instead of the outside world, they find only another dark corridor.
Later, the crew finds a sign pointing towards roof access. A solid wall is blocking them off.
Horror Struck: Members of the cast and crew at first dismiss the stories of the hospital as garbage, and even bribe a man to make stuff up because they don't believe they'll find anything. Then, when every story they heard comes true, suddenly they're all believers.
Humanoid Abomination: The ghosts look human, but their behavior, strength, and face contortions show that if they were, they aren't anymore.
I Want My Mommy: During a scene where Sasha is projectile vomiting blood out of her mouth she starts crying and desperately cries out that she wants her mom.
Meaningful Background Event: Early on, T.C. gets a phone call from his partner and child, so puts the camera down to answer it. As he talks, the wheelchair behind him moves by itself ever so slightly just as he says "Don't worry, monsters aren't real."
The final photograph Lance snaps (see Spooky Photographs below) displays the shadow of someone's hands pressed up against the window Lance is standing in front of.
Meaningful Echo: In the room with writing all over the walls, the patient inside had written, amongst other things: "DEMONS IN THE WALLS / DEMONS IN THE HALLS / DEMONS IN MY MIND / DEMONS YOU WILL FIND". Lance later starts screaming this mantra as he completely loses it.
Medical Horror: Dr. Friedkin, the famous surgeon who did tons of unethical experiments, including lobotomies mentioned at the beginning of the film is in the hospital with the ghosts still doing lobotomies with his nurses and tools. This is especially true since the ending strongly implies that the reason for all this ghost business is that he was taking part in somekind of Black Magic.
Never Sleep Again: The crew tries to stay awake in shifts, because whenever they all fall asleep, something bad happens to remind the cast they're being toyed with. Sasha is effectively "killed" in the end when a fog envelopes her and Lance while they sleep, only to have Lance awaken unharmed but with no clue as to where Sasha remotely is: taken by the fog while they slept.
Nothing Is Scarier: Somewhat averted, unlike most of the movies in its genre, the ghosts in this film get bored of being subtle after 15 minutes. There are still some nice tense moments where the viewer is waiting to see what's around the corner.
Houston's death is a result of this. We see him blunder down a hallway in pure darkness, unaware of the overhead camera's set up to watch, and is unable to see because his flashlight died. As he wanders down the hall, the viewer can clearly see Houston is the only visible entity on camera, but as he nears the end of the hallway, he is lifted off the ground and choked by some invisible entity. Houston is in total darkness and thus cannot see, so he begins swiping at the air trying to hit whatever's grasping him, but the viewer can clearly see whatever it is, it's apparently not only invisible, but intangible. Houston then gets violently launched down the hallway and dies from the impact.
Psychological Horror: They do have a lot of physical threats, but there are several indirect threats as well from sickness, lack of food, never ending corridors and no exits, culminating with the fact that time is passing very quickly and they have no light.
Psycho Party Member: Matt totally loses his marbles altogether and becomes insane. Luckily he doesn't hurt anyone, although he does indirectly cause T.C.'s death and also kills himself.
Room101: The hosptial in a general sense, and the operation chamber in a literal example.
Sanity Slippage: Pretty much everyone starts losing it as things get worse. It's open to interpretation if they're going insane because of the traumatic events they're experiencing, or if the hospital itself is supernaturally causing it.
Screamer Trailer: Several ghosts attack in the trailer, and at the end a bed is thrown and Sasha starts screaming.
Shadow Discretion Shot: Lance's lobotomy is not shown, you hear him plead sanity, then scream in pain before seeing the after effects.
T.C's death seems to be a Shout-Out to a similar scene near the end of [REC] 2 and the end is similar to the one in [REC].
Spooky Photographs: After the first few scares seem to have died down, Lance frustratedly starts taking photos in an attempt to grab some more paranormal evidence. He succeeds.
Stylistic Suck: Houston is only pretending to be a psychic for the purpose of the show... and it shows in his blatantly hammy and cheesy performance. Word of God states that Houston was based on a real-life "psychic" employed for a Ghost Hunters-style show.
Surprisingly Sudden Death: Virtually everybody, but T.C. and Houston especially, the former grabbed and vanishes, the second thrown through the air with no warning and dies.
Tongue Trauma: Lance finds a dismembered tongue on the floor. Blood dripping from the ceiling quickly alerts him to the tongue's owner...
Torture Cellar: The basic purpose of the operating room was to test lobotomies, as history will say the methods were often very cruel and the victims did not survive very long. Lance finds this out first hand.