Now, here are the tenants of my thoughts.
Belief Defines The Properties
This is important; many others have gained this insight. Compare the stories of the being from his first recorded instances and what you know today: people in the sixteenth century would either fear or respect the armoured knights. As a result, what they saw was, in essence, a knight with the skills to fight for a hundred knights.
Now, let us bring this to the modern day. The design of the suit is curious; I'm not sure whether anyone really fears the suit itself, and it definitely does not have the properties that armour does. The facelessness is simple enough: we are afraid of the unknown, of the darkness. A person you can't read is a personality you cannot identify with. But the suit is important; most people recognise the suit first, the facelessness second. I theorise that this may be the fault of the person who posted it to SomethingAwful; he created the being in the suit, and the suit become associated with it.
Now, the multiple arms. Those are an interesting point; after all, who would not love to have an extra pair of hands? My hypothesis rests on the fact that they are not hands; they're closer to hooks than anything. However, the belief that defines this property is the fear of something not being what you think it is. I've read some LiveJournal things, and I've heard that a lot of people mistake random people for their parents (I don't have this problem, since I don't meet a lot of random people who's skin is a patina of burns) and fear doing this; the same applies here. The facelessness is contributed to by this; from behind, it could easily appear to be a normal man from behind. The hooks and the facelessness are in place to reveal that it is not what it appears to be.
One could consider the facelessness, the hooks and the suit to be a modern equivalent of the armoured knight it once was. The armour could be thought of as a uniform; people wearing it became uniform. The helmet hid it's face, providing the facelessness aspect; the tales that its arms were lances became the hooks. The suit, of course, is uniform of sorts and thus replaced the armour.
Now, some would pick the hole in the theory that Victor Surge would not know about the monster; this is correct. I theorise that the being we call the Slender Man might not be a single being, but rather a collection of beings, but no two are alike. This may be the reason why the knight became the businessman; it's possible that they may not be the same being. But both the knight and the businessman have one connotation, and this takes me onto my next point.
But that is for another day, I think~! I had fun typing this, by the way~!
Also: this family has a nice set of candles. I'll be sure to burn them down to nothing by the time they're back next week.