Hi, I don't see an update to sys_journal_field when an idea is commented, so triggering a notification from the idea table would not be straight forward - unless someone else provides a better answer.
But checking sysevent, I see a sysevent for im_idea_activity which is linked to the table 'sn_actsub_activity' and looking at this table it appears to have links to the idea and the added comments.
I have not tested but I would think you could created your notification based on insert of sn_actsub_activity where when verb or activity type matches your requirement (IE comments), and then dot.walk to 'Object' to get the comments detail, then use a notification email script to lookup and populate the details from the idea record via addl_sub_obj2 and addl_sub_obj2_name
