The disproportionate dimensions of his body and the lack of a face seem to be the biggest reasons. That and him messing with the audio and video capabilities of cameras always gives the guys making the film leeway to make the sounds and visuals as creepy as possible and just explaining it away as "Slenderman did it."
AAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAGGGGGGHHHHHHHHHHHHHHH!!!!!- But seriously when your just looking at him; yeah; he's not all that... but when he's chasing you through the woods and you DON'T know what he looks like; yeah... that can freak a person out XD... I've seen the entire Marble Hornets series; trust me... When your first introduced to him in that setting and shit... Its scary as fark XD...
Heh, I just played the Slender game and I can see why it freaks people out, but really it's not as bad as a lot of people make it look. XD A lot of people say that the ambience is what makes the game really bad, but I like it--reminds me of Silent Hill. Marble Hornets didn't scare me, either; it kinda bored me to be honest.
I know all the talking and speculating in MH can be boring (its the reason I stopped watching the series XD) but the few moments that HE appeared; especially one of the first; were just so "OMG WTF is that?!" that it... I dunno... I must say, Slender man was actually the first thing to scare me in SUCH a long time... I grew up, from 7 on, watching the classics of the Horror genre... And these days I'm not scared by anything, which is why I quite enjoy Slender man... It gave me the rush I'd been missing for so many years XD... I can't even remember what the last good scary movie for me was... Mothman Prophecies maybe? I can't remember, lol... So Slender man is fantastic for me cause it gives me that fantastic adrenaline rush I'd been missing for so long.
I don't know if you've played the game or not ([link]) but for such a simple game it's definitely got everything needed to make it scary. Hmm...I'm not sure if I've ever really been scared by a movie before. We did watch one in high school that was pretty good, but all it did was make you uncomfortable at most. It did have good suspense, though; I wish I could remember what it was called.