Almost all Slender Man based ARG's have video sightings of the Slender Man accompanied with severe video and audio distortion. While it is not known for certain what causes this, EverymanHYBRID may explain it as radioactive interference by Sigma Radiation. The distortion first appeared in MarbleHornets, the first video ARG. Effects on still photography have yet to be seen, but the Original Mythos shows there to be no distortion.
Rarely in major ARGs does Slender Man act aggressively. In EMH, MH, TT and most other ARGs, Slender Man acts as more of a passive observer, if not an intereference or stalker. On the rare occasions he does attack, the video images are usually massively distorted or cut out entirely.
Some video ARGs number their video submissions as a series of some type. Started by MH, they are as such:
MH - Entries
EMH - Episodes (dropped early into the series)
MLA - Video Journals (dropped early into the series)
TT - Submissions (not in use anymore)
DH - Log Entries
The idea for Proxies was actually made by the Slender Man community to describe similar traits to different people in the ARGs. The first to set the trend was totheark, and Masky of MarbleHornets (although he is not actually a proxy). DarkHarvest00's Unnamed Proxy continued the same actions of Masky (mask-wearing instigators), and KindVonDerRitter was the same as totheark (posting distorted and cryptic videos to YouTube in response to the characters' videos).
EMH continued in a similar direction with their HABIT character, and their hidden videos aren't dissimilar to the videos of totheark. TribeTwelve's Observer also inserts videos, like EMH's HABIT, that are similar in style to totheark.
The style of cryptic messages, YouTube videos, twitter usage, and hacked or hidden information, is common across slenderblogs as well.
The Operator Symbol
The Operator Symbol is the symbol of Slender Man, referred to as the Operator in MarbleHornets. It appeared in the totheark videos and around Slender Man. It is commonly assumed that this is the case, but in reality we don't know for certain what the symbol means or is for. It was adopted by many slenderblogs and appeared in TribeTwelve and DarkHarvest00. It has become the popular symbol of Slender Man.
The Sickness refers to the common trope of Slender Man's presence causing illness and a number of symptoms. The symptoms start off common, such as coughing and fevers, but eventually grow more drastic, such as coughing up blood. IT SHOULD BE NOTED THE SYMPTOMS CAN ALSO BE CAUSED BY REAL LIFE DISEASES, AND ANYONE EXPERIENCING THEM SHOULD SEE A DOCTOR.
College Age Victims
While most of the time Slender Man goes after children, in most AREs and ARGs, Slender Man targets young adults. This is likely due to young adults being more fascinated with Slender Man, typically being better actors, and having the equipment to make the series in the first place.
For more tropes, see TVTropes article on the Slender Man Franchise.
Sorry folks, their list is too long to put here.