The feature is there in IF1, sold! It's even got its own technology! RSS!

IF2 already has an extensive notification system, which will be RSSable as well.
To defend my obviously brilliant idea:
It wouldn't be "Okay, I commented you, you MUST comment my stuff now", it'd be "wanna trade?". It'd be completely with the consent of both parties. I'd envisage it being between people who know eachother already, even on a superficial level, otherwise how could you trust their comments would be awesome enough?
When you have a large number of users and pieces, it's going to get to a stage when things are going to go unnoticed, it's getting there now even. On DA,
some people go around one-word commenting or faving purely to get exposure. This culture is not what I want, and I don't imagine it's what most want, especially on a writing site.
But it's impossible to quash that problem just through social pressure, you have to contain things too. You can't just stop a river, you have to reroute it. Channelling people's desire to get comments/etc into them leaving meaningful comments of their own would be better than seeing "Great!" all over your comments page.
To summarise, people will whore themselves out whatever I do, the most I can hope to do is make it useful. Perhaps I'll make them disable-able...