I often wonder what kind of thought goes through a person's head before they do an IaMa on reddit. You need to be either crazy, stupid, crazy stupid, or Neil Degrasse Tyson to think "Hey this is gonna be awesome!"
Still considering what happened to Woody, I would think that incident would give a few celebs and PR managers a moment of pause to think "Is this really worth the risk?"
On the other hand, generally the dark side comes out of Reddit only when people are obviously (ab)using r/IaMa soley to benefit their current PR campaigns.
Honestly, as harshly as reddit came down on Harrelson for that, it was entirely avoidable with even the most basic preliminary research. It's hard to feel bad for Harrelson, and it's similarly difficult to imagine that savvy PR types would be put off by the response. It's pretty easy to see what leads to a successful AMA, and not particularly difficult to prime your client for it.
It was 'Ask Me Anything About Rampart'