Comments are what gets most people to sign up for accounts and be part of a community.

Absolutely. How do you increase spontaneous interaction then? Certainly can't force anyone to comment by choking a rule down their necks. It would defeat the purpose.
Facebook has some kind of badges in Groups, namely "Rising Star", "Visual Storyteller", "Conversation Starter" etc. The system awards these badges to the people who regularly post in the group, initiate discussions.

Maybe something like that can be implemented on hive for comments. NOT for how many comments they have made over a period of time, people will start to spam one-liners—"wow nice thank you" then.
Rather the system can analyze comment threads and replies and re-replies and then associate certain badges besides their usernames, not just on their profiles.

They will keep on interacting for keeping their badges even though they don't have any monetary value and newcomers will see them and will try to attain that social status.

Inducing the feeling of envy is the most successful tool to make any social media grow.