What I want is a variation on a feature which already exists. If you start a topic, you can click the checkbox to receive email notification whenever the thread is updated (by somebody other than you). I'd like to get that option whenever I contribute to a topic. Once I've contributed to a topic, I may be as curious to find out what others have to say as the person who originally started the topic. And this is a feature which should scale OK; I think most topics don't have more than a dozen contributors, and maybe a hundred, tops. The only problem I can see with this feature is that people might then add a reply to a topic which doesn't actually have any useful contribution, just so that they can get email notifications in the future. That would be a pain in the butt.