Something I'd really like is the ability to search through my social news comments/submissions. Will websites like Hacker News and reddit be added to the list of indexes?
Have you talked to the guys who run http://searchyc.com about working with their data?
I use comment feeds to archive my own submissions into a feed folder on Thunderbird, makes it quicker when I can't remember a link or resource properly but know I've commented on it before. Also I have a vague idea that it might be interesting to look back on.
It means I usually try to quote or make it clear what the context of a comment is to make it a more readable resource if anyone should ever want to do that.