I was just talking with another designer here at work, and one thing she said is something that is vital to ARG design:

It is important to let the players have the narrative of the story.

Pushing the narrative, especially through meta-communication (asserting excessive control through the in-game conduits, or through out-of-game methods) can have a pretty distinct chilling effect on the immersive nature of the experience.

(Maaaan, I had several more paragraphs written here, but I just deleted them because I can’t make a succinct enough point, and there are dudes jackhammering and sawing right outside my window here in the office. Anyway, I’ll just leave the basic concept note there, and if I come back to it, I come back to it.)