I have just installed Brian’s Threaded Comments for WordPress 1.5. I have been looking for something like this for a long time, and I could never get it to work quite right on WordPress 1.2.X.
You can toggle the visibility of certain threads by clicking on the [T].
The only problem I can see with this is that more “sub”-comments that are made, the narrower the columns will be. That may become an issue for sites that receive more comments than I do, but that should not be a problem here.
Here is what the first comment in a thread will look like.
Here is what a “sub”-comment in the same thread will look like.