I think there should be a middleware layer for posts and comments, where you can write a number of custom routines.

func onNewPost( (user, post, commitFn, rejectFn) => {
   if( user.karma < 100) {rejectFn("Not enough karma to post here.")}
   if( post.length < 100) {rejectFn("Post is too short.")}
   commitFn(user, post);
});