A few years ago I was looking at how hidden comments might be more discoverable. As a result I talked with the developers about implementing a change so that when clicking "show X more comments" the focus would change to show the oldest newly revealed comment. That way readers would not have to scroll through the entire list of comments to see what's new. To see it in action, please click on this image:
Then compare it to the no focus version.
Of course I forgot to do the final step and roll that change out to the sites. Fortunately, the developers working on responsive design noticed the site setting wasn't getting used and asked me if we still wanted it. Better late than never?
I think it works pretty great for the relative edge case of posts with tons of comments. So I turned it on here on Meta Stack Exchange for your enjoyment. If the feedback doesn't turn up any critical problems, I'll push it out to the rest of the network.
To find posts that have hidden comments, I recommend looking at highly upvoted or downvoted questions. While there, marvel at the fact that each extreme required a bunch of discussion with our lawyer.
At any rate, let me know what you think of this change.