Basically, when leaving a comment I'd like to see the ability to check a box to subscribe ONLY to the comments that are in responses to the comment I just left. I often won't subscribe to comments (especially for very popular blogs) because I don't want to be inundated with emails. Frankly, I only care if someone is responding directly to something I said. Well, sometimes I care about all comments, but it would be nice to be given the choice.
Also, I assume this would only work with nested comments.
I don't even want this as a website owner. I want it as a reader.