There should be the option to subscribe to an idea via email or rss (and maybe even xmpp). Notification is given to everyone subscribed when a comment is written on the idea. You could automatically subscribe people via email when they write a comment, since they have to be logged on to comment and must provide an email address to get an account.

It is difficult to have meaningful conversation if you don't know when someone is speaking to you. Automatic subscription would mean people can ask previous commenters or the original submitter of the idea a question, expect a response, and know the response when they get it. Currently you have to regularly check everything on your user page, which doesn't scale well.