These are what we call the 'YMMV items.' Things that some people find in this work. We call them 'your mileage might vary' because not everyone sees these things in the same way. This starts discussions in the trope lists, a thing we don't want. Please use the discussion page if you'd like to discuss any of these items.
Even Better Sequel / Sequel Itis: Or remake, rather. Slender: The Arrival has been shown to do functionally everything better than its predecessor. On the other hand the plot has been criticized, it's very short and is considered by many an Obvious Beta.
Fridge Brilliance: It's only natural that the Slender Man would catch you immediately after you get all eight pages. After all, with each one that you acquire, it gets progressively easier for him to find you.
Good Bad Bug: Players have discovered that Slender Man is essentially locked into place if he's in your field of vision whether you can see him or not. People have exploited this by going into the woods then when Slendy shows up, they position themselves so that Slendy is blocked by a tree. This leaves Slender Man standing still in the woods like an idiot while the player sits unharmed.
This has been fixed in the remake. If you can get into a position where you can see him without the static appearing, you can see that he's physically moving (and he walks as fast as you can run). And if you manage to get him stuck on something, he'll teleport off to the side of your line of sight. He does occasionally just stand there, but on those occasions, he's either not hunting you down, or he's blocking the only exit you have.
Strangely, in the remake, you can put your back to a wall and stare into the woods for long periods of time without Slender Man showing up. While it would be unwise (for obvious reasons) to rely upon this as a tactic, it does provide an immense psychological comfort.
Hell Is That Noise: The slow, foreboding, rhythmic beat that starts playing once you've found your first page. It grows in intensity as you gather more.
Hype Backlash: The community of the mythos seems to be displaying some of this towards the game.
A lot of the community's backlash has less to do with the game itself, and more to do with its fans. A few people have looked into it and discovered the rest of the The Slender Man Mythos, but a lot of people who were introduced to the Mythos through the game think that the game created the Slender Man, and some will even go so far as to say that Marble Hornets stole from Slender.
Most of this really has to do with the type of game it is. It is an atmospheric piece,and as such, is not for everyone.
I Am Not Shazam: Some fans who have discovered the mythos from playing this game assume that the monster is called "Slender." He is not. His name is "Slender Man." The game is called "Slender." Getting this wrong is a surefire way to make mythos fans want to strangle you.
Not to mention all the people who have somehow deluded themselves into thinking Slender Man's name is one word.
Moral Event Horizon: It's strongly implied that the Slender Man turned Kate into the Proxy. Ouch. Needlessly malicious, much?
Nightmare Retardant: The secret "Twenty Dollars" mode from one of the older versions can take the edge off if you've been playing for too long. It's entirely the point in fact.
The model for the Slender Man looks like a blow-up doll or a mannequin. It can take away a lot of the horror if you see him in full view. Thankfully, he gets a considerably more intimidating model in the remake.
Pop Cultural Osmosis/Critical Research Failure: An infamous example. Like the I Am Not Shazam and Hype Backlash example above, the game's massive popularity had misled many people into thinking this game invented the Slender Man, even though the character originated from Something Awful and the game's inspiration comes from Marble Hornets. One particularly cringe worthy case involves Teens React claiming that the game was the inspiration for Marble Hornets when it was really the other way around. Later versions have since been addressing the issue by putting a The Slender Man Mythos option on the start menu.
Scrappy Mechanic: The sprinting system in the early versions got the most criticism, particularly since the Player Character walked so slow, could only run a short distance, and his/her maximum stamina would deplete permanently every time she ran. Run too much at the beginning, and the game could end up Unwinnable by Design. Later versions have since been addressing it.
Serial Numbers Filed Off: Milkstone Studios S.L. basically took Slender, changed the pages to tape recorders, and released it on the Xbox Indie Games label as White Noise: A Tale of Horror.
That One Level: The abandoned complex. Tight corridors, low ceiling, maze-like halls, few exits, blind corners, dirty linoleum, and nowhere to run. If he finds you in there, you're screwed.
For a while it seemed like Slendy couldn't spawn inside the building due to a code change in 0.9.4 that prevents him from appearing inside of objects. Then someone found out - he just appears in the hallways. Directly in front of you.