Under Account > Settings > Notifications there are two options (in the "Activity from subscriptions" section), for receiving notifications for comments or status changes on Ideas I'm subscribed to.
If a person wants to receive notifications for any comments added on any Idea within an Ideation, they either have to add a comment (before anyone else starts to comment) or they have to "subscribe" individually to all new (and old) Ideas that get posted.
In the notifications options, please add one or two new options as shown below (bold italics added just to emphasize what's different about the new options):
Of course, given the existing Ideas notification options it might make sense that these new option(s) would be turned off by default.
If any options like these do get added, please also add a mention of them on the "How to receive notifications" page of the Ideation Wiki