I agree about all of that except for contrarianism (and yes, I'm aware of the irony). You want to have some amount of contrarianism in your ecosystem, because people sometimes aren't satisfied with the hivemind and they need a place to go when that happens. Sometimes they need solutions that work where the mainstream answers wouldn't, because they fall into a weird corner case or because they're invisible to the mainstream for some other reason. Sometimes they just want emotional support. And sometimes they want an argument, and there's a place for that too.

What you don't want is for the community's default response to be "find the soft bits of this statement, and then go after them like a pack of starving hyenas tearing into a pinata made entirely of ham". There need to be safe topics and safe stances, or people will just stop engaging -- no one's always in the mood for an argument.

On the other hand, too much agreeableness leads to another kind of failure mode -- and IMO a more sinister one.