Abandoned Playground: The Rainwood Day Camp Which is also a playground of sorts for the Slender Cult.
Big Damn Hero: Jeff, saving Chris and Alex from certain death at the hands of the Slender Cult.
The Blank: Besides Slenderman, Kind von der Ritter wears a mask that evokes this. The cultists masks have human-like features (they wear Guy Fawkes masks painted completely white), but from a distance it's closer to this.
Expy: Kind Von Der Ritter is sort of a mix of Totheark and Masky. He wears a featureless mask that makes him look like a short version of Slender and a rather odd outfit. He also sends creepy yet helpful messages to the heroes, and behaves almost violently when actually confronting them.
Eldritch Location: The Fourth World, which is either the location of the Eldritch Abomination Slender Man works for, or is the location of the Slender Man's true body (with the suited humanoid being a mere projection). The only type of energy that the Fourth World has in common with ours (the Third World) is Life Energy ("vim"), so it's got to be a pretty mind-bendingly bizarre place.
Fate Worse Than Death: The Order is not allowed to kill Chris or Alex, as their prophecy dictates that Slender Man must be the one to do it. However, they have no trouble threatening Chris and Alex with one of these if they don't start minding their own business, saying that first they will kill their families, then lock them up and torture them until Slendy finally comes for them, at which point they will be begging to finally be put out of their misery. They've already started the first part.
Genre Blindness: These guys are rather enthusiastic to explore Slasher Film locations at night. They'll enter the locked house of a guy who they know had problems with home invasions, explore deeper into a creepy forest when they hear a branch snapping, and jump into boarded-up derelict buildings.
Greg, especially in his final log. One does not simply leave the Order
Government Conspiracy: Some portions of the U.S. government are aware that Slender Man is more than just a Serial Killer, and are (or were) actively doing research into the paranormal "science" behind him. One of them probably posed as one of the investigators into Greg's murder, since one investigator that interviewed the characters refused to say which agency he belonged to, much less give out his name.
Grievous Harm with a Body: Slender Man seems to throw a cultist at the car containing Chris, Alex, and Noah. You can hear him screaming as he flies through the air, and then see him impact the hood.
Haunted House: Chris initially thinks his house is haunted by a ghost. That may have been preferable.
Heroic BSOD: Chris seems to get one when Alex picks him up from the police department after a gang of cult members break into his house and stab him.
Meaningful Name: You're going to want to turn up the brightness and contrast on your monitor before watching. Most of the early episodes take place in a house suffering from frequent power outages and inexplicably closed curtains, or at night with all the lights off.
Mind Screw: The end of Log #19 Slender Man leans out from behind a door frame, but then at the start of 19.5 they act like it didn't happen. Add in the distortion at the end of 19.5 and things are definitely getting screwy.
Organ Theft: Alex and Chris find a bags of internal organs several times in the series.
Police Are Useless: Towards the beginning, the characters don't hesitate to call the cops whenever something strange happens, but nothing ever comes of it. They come to quickly realize how much this trope is in effect. This comes to a head when Chris's house is invaded by at least ten cult members, and Chris is stabbed before managing to lock himself in his bathroom and await rescue from Alex. The next video starts with Alex picking Chris up from the Police Department with no helpful results.
At least one detective is a member of the cult, and the boys speculate that he's the one enforcing this trope.
Radio Drama: At some points in the videos, due to choices in cinematography and lighting.
Renegade Splinter Faction: The Order's Sixth Enclave, considered heretics by the rest of the Order for being willing to ignore the prophecy and kill Chris and Alex themselves.
Sacrificial Lamb: One of Chris and Alex's friends, who was being stalked by Kind von der Ritter and Slender Man.
Seeker Archetype: Chris wants to find out what's going on so much he tends to throw caution to the winds. He'll pester and bully his friends into following him, leading to several What the Hell, Hero? moments.
In Entry #3, we find out that there was a detective on the scene after Log 14-2 who seems to be quite familiar with a certain "Tall Man". At first viewers theorized he was Zeke from Seeking Truth, but it seems he is an original character. Possibly the name is a shout out: Detective Amsel. Lookfamiliar? The Amsel family plays a larger part in the series' myth arc.
The Operator symbol from Marble Hornets appears a number of times. Along with a label-less bottle of pills in a restroom of a creepy house.
Time Travel: In one episode the heroes spend an entire day and some of the night driving to and exploring a government facility. Just as they return to Chris's house, they travel back to just a few minutes after they left. There is also some of Kind Der Von Ritter's abilities.
Greg's Log #3 reveals that both Jessie and Greg were part of the Slender-cult at some point (and the former probably still is).
Log Entry #30. Alex calls Chris and tells him that he found his family murdered. Chris returns home and finds his family murdered too. The two of them are now on the run with Heather with almost no resources between them.
White Mask of Doom: Kind von der Ritter. The cultists wear what seems to be a white-painted Guy Fawkes masks.
Word of God: Alex and Chris have become some-what regulars on the Around the Campfire podcast, where they share a lot of behind the scenes details. Their first appearance was on episode 2, with Chris returning on episode 3 along with Tim from Marble Hornets.