Here's an example of a HN comment with 200+ comments:

http://news.ycombinator.com/item?id=2530963

Here's the same thread in my layout:

http://threadit.kaizenrails.com/discussions/839

I have to admit, I don't immediately get it. Kudos to you for hacking on comment thread presentation and interaction, however. :)

The arrows let you cycle through the replies of the above comment. This way, instead of using indentation to indicate a parent or ancestors, you simply look at the comments above. I find it easier than following indentation or indentation lines to mentally juxtapose a comment with its parent and its ancestors.