Not sure if this falls under the general topic of social network like features or not but I thought I'd throw out this thought...
The ultimate goal of the GCD is to index all comics and hence you need to motivate the user base to actively contribute. Other sites have successfully utilized virtual awards be it badges, karma points etc to motivate people to contribute. One specific example is Joel Spolsky's StackOverflow based sites.
The idea being that you get badges for specific achievements, indexing a book, adding a cover, reporting an error etc and points for ongoing activities. These would be displayed on user profile pages and perhaps a leaderboard type system displayed on the indexing pages (which would make it semi-private).
Users could also use their "points" to put bounties on specific activities to help them, eg: give a buddy X points for looking up a issue credit or indexing that last issue in a run.
It would take some work to determine the set of rules to be used and then the infrastructure to properly account for the activities and then properly present them but I think it would introduce another incentive to get users to contribute to the system.
Phil