Alternative Character Interpretation: A fair number of fan theories involve Slendy himself not being evil or malevolent. Either he's a lonely figure who unintentionally curses those he comes into contact with in his efforts to make friends, a harbinger of doom, or even an incompetent guardian trying (and failing) to save people.
A viewer occasionally wonders how the various ARG characters find the time or motivation to update their various journals, especially while being chased by the creature himself. However, if played right it can lead to both Fridge Horror and Fridge Brilliance.
And then there are the videos that show things that should have never made it past the editing process, or should have merited a complete re-shoot as bad as they were. This is actually addressed in the Everyman HYBRID universe: not only is it implied that the protagonists can't see the screwy bits in "normal" videos, but it's stated that they can't see the really weird ones at all. Why do people keep telling them about something called dot-dash?
Many people believe that Slendy exists because people think about him. Many also believe he only targets people who believe in him. If he succeeded, would he die?
Although this ventures more into WMG territory, this could lead to a stroke of Fridge Brilliance: Perhaps that is his motivation towards his acts. Maybe he's not really as hostile as he's portrayed. Perhaps he simply wishes to stop existing, to rest.
Or maybe, he doesn't exist, but he wants to. And what better way than something as innocuous as the internet?
Hype Backlash: A result of overexposure. Slender is almost an exercise in how quickly something genuinely interesting becomes cliche and predictable with its fans hyping how scary it is. Nowadays when the Man is bought up, it'll be regarded as Snark Bait more often than not.
Just Here for Godzilla: Because let's face it, some of the stories can be downright boring without Slender Man in the picture.
Memetic Molester: Slendy apparently likes to hang out around children, as the earliest photoshops show. A common theory suggests that any adults Slender targets are simply children he didn't get to finish off. Make of that what you will.
This comic provides one possible explanation. It...doesn't really make things any better.
Memetic Mutation: Slender Man first showed up in a "make normal photos paranormal!" photoshop thread. After the first few Slender Man pics, he took over the thread. Appropriately enough.
There is also an ongoing joke that Slender Man is stalking Alex and Jay because he wants their $20 and that he would go away if only they gave him the money. Word of Gods state that they are sick of that particular joke.
Moral Event Horizon: The bloggers get quite angry when Slendy starts using children as proxies.
Narm: The blog No Escape From Reality (now deleted) started out as this, but then became infamous in the Unfiction forums after the author tried to turn his blog into part of the Everyman HYBRID series. Apparently, the author is still on Unfiction, and considers it an Old Shame of his.