2. Most of the stories involving Slenderman are told in the first person. For as long as stories have been told, people have used the first-person perspective to try to make them more immediate. From stories told through letters and diary entries on down to the found-footage horror boom, trying to make a story scarier by placing the audience inside of it is a technique that is incredibly effective when done right.
The bulk of the stories about Slenderman are told through blogs and found-footage video. As such, the tales about him have that extra-creepy vibe of "oh man, what if this happened to me?" We live in a society in which everyone has a camera with him or her at all times. Can you imagine how horrifying it would be to look back at your Instagram uploads one day and discover that something with overly long arms had been in your life for the past few months and you'd never even noticed it?
Hollywood may be ruining the found-footage genre, but there's a generation of storytellers on the Internet using modern technology to scare the hell out of people with next to nothing in terms of budget.
3. There are no rules. When do werewolves become werewolves? Everyone knows that it's when there's a full moon. Except that in The Wolf Man (1941), that wasn't the case at all. That didn't happen until the sequel, which is when the idea really took root in our collective conscious.
Right now, only five years removed from creation, there are still no hard-and-fast rules about Slenderman other than that he wears a suit. Sometimes he has tentacles and sometimes he doesn't. Sometimes he's only after children and sometimes he terrorizes guys making workout videos. My favorite ability of his is the power to shift people through time and space, as shown in the above episode of Marble Hornets.
Because he can be anything, people can make him into the creature they need to tell the story they want. Slenderman isn't saddled with hundreds of years of baggage like vampires or market oversaturation like zombies. At least not yet, anyway.
He is, after all, only five.
