"the mod queue seems a trifle arbitrary"
Some articles have been marked for hand-moderated comments, most not, eg this one, which is why your comment went through immediately.
Articles and users on manual-moderation mode have to wait for someone to be free to clear the queue; certainly I've seen the queue averaging 10-40 posts.
If you find yourself in the queue, it may be because you posted a correction as a comment, or had a comment recently rejected/removed. That'll put you in the queue for a while.
C.