I played the game and while I did find it scary there are a couple of things that, if fixed, would make things far better.
1. Something has to stop Slenderman from spawning right on top of you because you're directly on a corner/tree, it happens a lot during play and even in let's plays and the like. Yes it's scary, but it's also an unstoppable, uncontrollable insta-kill which is complete bullshit. If the previously mentioned method of removal is something they don't want to do because having him spawn on you like that is too good of a scare they can always make it so that the capture time is retarded (I say that in the literal context, not in the context of "this is shit"), allowing for a players pants to feel a great deal heavier whilst also giving them a reasonable chance to escapte.
2. The building/s in the game are really featureless (I had the game on the top graphical settings in case there were some decorations that are removed in lower settings), having nothing but chairs in them, although the tiled design of the insides (I only found the one building, I'm guessing I missed some, but this will be addressed more in my next point) give me the impression that they are public restrooms. The build of the game wasn't yet 1.0, so I assume that they're going to add the shitters and sinks in later.
3. A real lack of landmarks between the REAL landmarks (the REAL landmarks being like the "public restroom", the gigantic tree and the "Gas Storage Center" [I don't know what that area would actually be called, sorry], although there's probably more that I didn't find) which are spaced apart a great deal. I know some people are wanting to say "You're meant to get lost, you're not meant to be able to just know which way to go to find the papers", a sentiment which I agree with, to a point. What I'm referring to are more simplistic things; like noteworthy rock formations, Benches or water fountains and differences between them (say a lot of benches get placed around the map to decorate it [This is a park after all], where, for example, a bench has fallen into disrepair, a tree has "Harry loves Sarah" or something engraved into it or a fountain has received a gigantic dent which causes it to lean to its side), this allow players to become lost and think "shit, where in the park am I?" as they do now, but if they stopped and took a look at these telltale signs (stopping being rather dangerous) they could grasp their surroundings and head to the next location they think may have a paper(if they already know the park, meaning they've probably played a few times and been killed or having been running around for a lengthy period of time and have just recently noticed things, the former being far more likely than the latter). Not only would this stop things from being an almost complete navigational nightmare, but it would make things a lot more believable, at the moment there's trees, trees and more trees and a couple bare areas between locations, it's not something I can really buy.
4. Walkpaths. The Game needs walkpaths. This is a park, people go walking in them. The walkpaths should go between the major landmarks of the park, perhaps in a large circle (this kinda crap is in real parks, people play in them, walk dogs and other piddling shit that someone who actually treads into the sunlight would know more about than I), a long pointless walkway that loops around and shit (you silly-ass joggers), that kind of thing. You might be thinking "If people follow paths then they're less likely to get lost and can easily move around areas", this is true, so a mechanic would need to be implemented that made it so that Slenderman because a lot more "aggressive" (for lack of a better term) if a player is very close to the path, making players question whether they want to chance becoming a lot more easily lost, but having a less aggressive Slenderman (the current aggressiveness, minus the shitty spawn atop players thing mentioned in point 1) or dealing with a hyperagressive Slenderman (I don't have many suggestions for this, but I guess some simple ideas would be that he spawns a lot more frequently and closer to the player) and being able to more easily reach navigate the park.
As I said at the start of my post the game is good, but it's really lacking in a couple of areas, mainly just making me believe that this is a real park/forestry area, however the game isn't a 1.0 release yet, meaning I assume a lot of things will be added at a later date to make things more believable (like the "bathrooms" I mentioned), something that their patch notes imply they've done in other areas in the past, like adding a wind effect on the grass and trees, a massive improvement over having nothing but static and unmoving flora, it's simple but adds a lot more life to things. However, if it's left as is, it's still a decent game and with it being free it's totally still worth a go.
NOTE: I avoided mentioning graphics because I think that they're perfectly fine, they just need more details (example of what I mean; a lot of games allow players to make custom maps [I'll use L4D as my example, but don't worry, even if you haven't played it just think "Zombie Apocolypse"], but this is a zombie apocolypse, there are zombies filling the streets, but some custom maps are too "clean", as though someone just got through scrubbing everything to its brightest and cars are parked properly; everything looks like something out of a fucking postcard. There should be cars thrown about, turned over, broken windows, corpses in the street, blood and bile smeared about, broken windows in the buildings, messages scrawled on the walls [things that are all addressed in the official maps, because they know that this is something they have to do to make the world real]. This sort of thing doesn't require any improvement on the current graphical standard they've set and would actually be very little work.
EDIT: I just read a bit about Slender Man and apparently he takes his victims to forests, but my comments regarding landmarks and the like is still true, it would simply change the type of landmarks available, like making the walking paths much much direct, maybe one or two being long and scenic (spelling?), maybe a stream, a main road going through (considering some of the landmarks and that it's a forest I'd downright expect a real road going through at least part of the area with some short dirt roads breaking off to reach landmarks by vehicle (with Slenderman becoming even more aggressive to compensate for the easy navigation of a road and story wise because you're on a fucking road, can't have you potentially running into help [despite that being unlikely], but to make sure you of didn't just run down the road out of the game you'd make it so that approaching the exit would cause the screen to do the static thing, building as you get closer to the edge of level barrier, with the player instantly being killed by Slender Man if they cross it, this being far better than the alternative of sealing off the ends of the road with a truck, fallen trees, etc because it also gives the horrifying impression that he's keeping you in there with him.
. always waiting to hear him saying "mr. freeman". 
