WordPress recently introduced threaded comments. That way you can respond to a comment midway through a thread and your comment stays with the one it is responding to. It's a nice way to maintain threads of sub-conversations.
I've postponed enabling them for some time because I didn't like the way they looked by default and knew it would be a bit of a challenge to style them the way I wanted.
Meeting the Challenge
I was certainly correct. Styling threaded comments to obtain the look I wanted, and with the restrictions imposed by the new display method, was one of the more difficult css hurdles I've overcome. But I'm very happy with the result.
The impetus to final take it on was the release of a new plug-in that allows the admin to mark a comment as "featured" or "buried" which then adds a hook that lets you style those tags.
What Does That All Mean?
Comments will display in alternating backgrounds of white and light gray (almost bluish) whether they are linear or threaded comments.
Author comments (comments that I make in a thread) will have a red background.
Featured comments (particularly useful, helpful, or insightful comments) will have a light green background to make it stand out.
Buried comments (Not necessarily "bad" comments but those that don't really add to the thread, like "nice post" or "great idea" but don't deserve to be deleted) will be grayed out, reduced in size, and have the "reply" link removed.
Threads can go three levels deep. I arbitrarily decided that three levels was sufficient. Additionally, the width of the comments narrow at each level so anything beyond three levels deep makes the comments squished and elongated.
Here's an Example
I'll put up some bogus comments to illustrate what I'm talking about. I think it all came out very nicely. I hope you all like it.