Maybe it could tied to group permissions and/or post count?
I think we'll do this for 8.0. Right now, 99% of people don't have a group for new users with limited access, so 99% of forums, as soon as a user registers they are going to be able to post visitor comments. When we do 8.0, we'll add a new default group for say 25 posts, so everyone will have it set default, without needing to jump through any extra steps.
For now, I just want to make it so people can't just sign up and spam visitor comments.
