I agree that "nasty" insider information or insider opinions should not be posted.There's a big difference between expressing opinions and stating nasty, unverifiable rumors as fact.
I would avoid obscuring the easy "nasty" rule by distractingly talking about "verifiability" or factualness. The reason is that even if an insider knows that a fact or opinion is 100% true, that insider *still* should not post that fact or opinion on a public forum if it is "dirty laundry"--i.e., if even one team member or recruit would consider that fact or opinion to be private.
Our two biggest insider tippers, Ames and Doe, have given solid information, but both have also violated the team's trust in them. Ames by stealing Hall's thunder on his verbal, and Doe in this thread by revealing alleged internal conflicts. By doing so, they make the team look bad because now people know that the team has loose lipped people who don't respect privacy.
When I put together a team for a project, I would *never* hire guys who feel an urge to behave as Ames and Doe have done.