I have voiced my disagreement with this elsewhere, but I must reiterate that I am not in favor of restricting downvotes to comments you've replied to. Replying to a troll, for instance, is the wrong thing to do.

Good point. I retract that suggestion.

And the automatic upvote to one's own comments should be included under that limit.

The automatic upvote to one's own comments is slated to be removed, with the stated intent to normalize comment scores to a baseline of 0. There's an issue entered in LW's Google Code issue tracker for this already.