Hmmm that's kind of beyond the scope of communities and I am a little worried about the performance impact (more things to track) but it does seem like a worthy addition to notifications. It's true that in other medias when you write a reply then you are subscribed to the "thread".

Would you like to see all comments in reply to a post ? Only for comments right ? Eg if I were to reply to artgirl in your comment thread you'd like to see a notification like "howo replied to artgirl on your comment antisocialist/...."