I would definitely add FACEBOOK/TWITTER/OPEN ID LOGIN buttons for one fast click login, ( people tend to be very lazy in these days ) and like delicious or stumbleupon allow for bookmarking unlimited amount of sites for each user, sort of a private bookmarks with option for public display. Like LowryBigwood suggested recip link for bookmarks would be a big NO NO, also you need to disable unique links URL and somehow incorporate counter for how many users already bookmarked the same link.
Allowing submitters/users to populate submitted links to other sites in network would be a nice addition with one simple setting.
Having ability to post to incorporated blog and open forum with one login is another nice addition, but starter of this topic know this already
Things like auto tagging, rewards for comments and ratings, auto newsletter for registered users, extracting meta data from submitted links, everything that will make submitter / book-marker adding new link fast and painless.
Reciprocal links could be awarded with higher positions on your pages and / or additional listing on other sites.
Some of the stuff I already implemented into my sites, on some I'm still working.
Link lists are not dead, like UW stated in his latest posts, they just need little re-invention.
2c